9 biodiversitymyresource.phoenix.edu/secure/resource/bio100r3...240 chapter 9 biodiversity: how...

239

Biodiversity:How Diverse Is Life?

Classifications both reflect anddirect our thinking. The waywe order represents the waywe think.

—Stephen Jay Gould (1983)

Chapter opening photo A richassemblage of organisms lives deep inthe world’s oceans associated withhydrothermal vents.

OverviewLife on Earth is amazingly

diverse. In the last 250 years,

biologists have described

roughly 1.5 million different

kinds of organisms, and they

have only scratched the

surface.

In the 1950s, biologists in

the tropics discovered in the

tops of trees that are

hundreds of feet high a rich

assemblage of living things.

Since these organisms only

rarely and accidentally come

near the ground, they were

largely unknown. Borrowing

equipment and techniques

9

240 CHAPTER 9 Biodiversity: How Diverse Is Life?

from mountain climbers and those who build tall buildings,

biologists took to the treetops and found thousands of new species.

Their work goes on. When completely cataloged, these and other

tropical ecosystems may well add 15 million additional species to

Earth’s catalog of diversity.

The deep oceans, once thought to be largely biological

wastelands, are also proving to be surprisingly diverse. In the

1970s, rich biological communities were discovered associated with

geologic vents on the ocean floor. These vents spew hot, mineral-

laden water, which in turn provides energy and nutrients to

microbes. Feeding on the microbes were hitherto unknown fish,

crabs, clams, giant worms, and others. Away from the vents,

oceanic bottom muds don’t exactly teem with life, but even there,

extensive and unique life forms were found. The same surprises

awaited biologists who investigated the tops of seamounts and the

thin layer of transition between ice and water under the pack ice of

the Arctic and Antarctic Oceans.

More recently, in the crawl spaces between soil particles, a rich

assemblage of organisms has been discovered. A little too large to

be studied by microscopists and a little too small to be easily

noticed by those without a microscope, the so-called mesofauna

have quietly gone about their business largely unnoticed. Now

alerted to their existence, biologists are finding them everywhere.

Apparently, their numbers and diversity are extensive. Likewise,

new organisms are being found thousands of feet deep in rock

cracks and in muds and soils beneath lakes. Their life cycles and the

extent of their diversity are still being discovered.

How many species are there on Earth? Educated guesses now

run in excess of 100 million species, and biologists will continue

to discover new species for a long time to come. Such diversity is

both a blessing and a curse for biology. Each species represents a

unique solution to the challenges of life. The process of

9-1 How Did Life Originate? 241

understanding the breadth of such solutions is exciting,

challenging, and awe inspiring.

The curse comes in describing, identifying, classifying, and

naming these species. What is needed is an extensive data

management system. Ideally, it would arrange species in some

logical order, indicate how each is related to others, assist in

identifying them, be sufficiently robust to handle at least 100

million separate objects (species), and be “user friendly.” Biologists

who are not centrally interested in classification should be able to

get information into and out of the system with minimal training.

Fortunately, biology has, indeed, devised such a system. Managing

the system are two branches of biology: Taxonomy and systematics.

Taxonomy describes, classifies, and organizes organisms according

to their similarities and differences. Systematics is more

interpretive: It studies evolutionary relationships among organisms.

The first step in such studies, accomplished by taxonomy, is to

group species according to shared characteristics, which indicate

ancestry: The more similar two species are, the more closely

related they are. But all organisms, no matter how dissimilar, share

some characteristics. If taken far enough, the study of similarities

and differences and the study of evolutionary relationships shed

light on the very origins of life. Taxonomy and systematics start

with the story of how life originated.

Discovering Vertebrates Are biologists still finding any new terrestrial vertebrates, thatis, amphibians, reptiles, birds, and mammals? If so, where are they being found?

9-1 How Did Life Originate?

Today, there are millions of species living in virtually all of Earth’s nooks and crannies,but we can imagine that, at one time, in its far distant past, there was no life on Earth.Where did life come from? Let’s attempt to answer this most basic of questions.

Interest in life’s origins is probably as old as humanity. Indeed, ancient Greek andRoman scholars may have first proposed spontaneous generation to answer the questionabout the origin of life. It’s a question that plagued Darwin. “Life,” he answered, “may

Exploration

Exploration


have originated in a warm, little pond” (Figure 9-1). In 1858, when he first publishedThe Origin of Species, the implications of evolution on the idea of spontaneous gener-ation may not have been obvious.The idea, inherent in the cell theory, that all life comesfrom preexisting life had not yet been proposed. Darwin ducked the question and movedon to other arguments.

Where It All Began: Consider some hypotheses for the origin of life such as Panspermiaand hydrothermal vents.

Other scientists, too, avoided the question. Science does best with questions dealingin measurable, repeatable phenomena that can be tested, experimented on, and ana-lyzed. How can one do experiments on the origin of life? Best leave such questions inthe hands of philosophers.

Yet, other scientists were less reluctant. Besides, the question will not go away. Ifbiology studies life, its origins will continue to be inherently interesting. Definitiveanswers may never be possible.We will never be able to say,“This is the way life evolvedon Earth.” But we may be able to garner enough circumstantial evidence to at least say,“It may have happened like this.” Let this be our goal.

Early Speculations on the Origins of Life Lacked Experimental EvidenceIn the 1920s and 1930s, the Russian biochemist A. I. Oparin and the Scottish biochemistJ. B. S. Haldane wrote a series of papers that sparked contemporary interests in life’s ori-gins. They reasoned that Earth’s early atmosphere was considerably different fromtoday’s atmosphere. Where is the evidence? Astronomers at the time studied the at-mospheres of other planets in the solar system and found little free oxygen.Why shouldthe early Earth be different? Also, rocks that were on Earth’s surface 3 billion or soyears ago contain free iron. In today’s atmosphere, free iron quickly reacts with oxygento form rust (iron oxide). The absence of rust in those ancient rocks suggests Earth’sancient atmosphere had no free oxygen.

What gases would be present in the early atmosphere? Oparin and Haldane spec-ulated on an abundance of methane, ammonia, nitrogen, water vapor, and, perhaps, freehydrogen.They also envisioned a variety of energy sources present on primitive Earth.Earthquakes and lightning would have been more common than today. No free oxygenwould mean no ozone layer in the outer atmosphere to keep the sun’s ultraviolet radi-ation from reaching Earth’s surface. All those energy sources working on all those at-mospheric chemicals would have stimulated chemical reactions. In particular, aminoacids, the building blocks of proteins, which are the building blocks of cells, would arise—dare we say it—spontaneously.

Initially, Oparin’s and Haldane’s speculations were not well received.Will the idea ofspontaneous generation never go away? Besides, where is the evidence? How can therepossibly be evidence? Skeptics reasoned that the formation of amino acids would havetaken millions, perhaps billions of years to occur. Untestable hypotheses in the absence ofevidence are nothing more than idle speculation—little more than science fiction.

Early Experiments Spontaneously Produced Organic CompoundsThe speculations could well have died then, except for an ingenious experiment conductedin 1952. Harold Urey of the University of Chicago and Stanley Miller, a graduate student,built an apparatus that modeled Oparin’s and Haldane’s atmosphere (Figure 9-2).Theyused electric sparks to simulate lightning in a simulated atmosphere of methane,ammonia, hydrogen sulfide, and water vapor. Water in a flask simulated an ocean. Thewater’s evaporation and condensation simulated the water cycle.Amazingly, in less thana week, their water turned cloudy. Amino acids had formed!


1

34

5

6

7

2

Figure 9-1 ■ Darwin imagined that life (1) might have originated in some small, warm pond.Other biologists have proposed that life began (2) in hydrothermal vents at the bottom of theocean, (3) in heat-stressed ponds near ancient volcanoes, (4) in clay beds in estuary or bays,(5) in tidal pools, or (6) within bubbles of foam formed by ocean waves. Still other biologistshave held that (7) the precursor chemicals of life came to Earth on meteorites.


Others repeated the experiments with similar results. By varying the composition ofgases and other conditions, other carbon compounds appeared, including carbohydrates,lipids, the components of DNA and RNA, and other amino acids. Not only was it possi-ble to synthesize the building blocks of life in the proposed ancient atmosphere, but thesyntheses took place easily and quickly.

Could Oparin and Haldane have been correct? More circumstantial evidence accu-mulated. Astronomers found simple organic compounds in meteorites. Again, scientistswere initially skeptical and some even derided it. Maybe the meteor hit a bird on its tripto Earth. More seriously, as they streak toward Earth, meteors may pick up bacteria andother spores. Careful analyses of several meteorites found the questionable compoundson the inside. Other instruments remotely sensed methane deep in open space. Carbon-based compounds, once thought only to be associated with life, are apparently commonthroughout the universe.

Then the pendulum of technical opinion swung in the other direction.Astronomersand geologists became convinced that Earth’s initial atmosphere could not possibly havebeen as Oparin and Haldane had imagined. It was much more likely composed mainlyof carbon dioxide, nitrogen, and water vapor. In this atmosphere, still lacking free oxygen,organic compounds would be much less likely to form. Speculations continued: Perhapsmeteors or comets brought Earth its first precursor carbon compounds. More circum-stantial evidence accumulated: Fossils of ancient bacteria that were 3.5 billion years oldturned up in western Australia. At least 11 different kinds of fossils were found there,suggesting that life must have evolved quickly, in something less than a billion years.

Tungsten electrodes

Electricsparks

Condenser

Gasmixtureof methane,hydrogen,ammonia, andwater vapor

Boilingwater

Stopcocks for withdrawing samplesduring run

Condensedwater

Trap

Figure 9-2 ■ The apparatus used by Miller and Urey. Conditions inside the apparatusduplicated conditions of Earth’s early atmosphere. Gases contained no free oxygen, but wererich in methane, ammonia, and hydrogen. Water was heated in the flask at the lower right. Watervapor flowed through the tubes and past the sparking electrode, representative of lightning,which supplied an energy source for the reacting chemicals. The water condenser turned watervapor into droplets that flowed into the trap and eventually back into the flask. In a surprisinglyshort period of time, organic compounds, including amino acids, gathered in the trap.


What is the state of conventional wisdom today as to the origin of life? To paraphraseVirchow, “even though the strict proof has not yet been produced in every detail” (seeChapter 2), it must have been something like this:Almost as soon as Earth cooled to thepoint where oceans formed, precursor carbon compounds appeared. They were eithertransported by comets and meteors; were formed in an atmosphere different from today’s,or both. The first steps in the spontaneous generation of life had been taken.

The Next Step Was to Move Beyond Isolated Carbon-BasedCompounds to CellsJust as a pile of bricks is a far cry from a building, the presence of precursor carboncompounds is a far cry from life. How could even an ocean of simple carbon compoundsspontaneously organize themselves into living cells? Some of the most complicated chem-ical reactions must have occurred spontaneously. In today’s world, enzymes invariablycontrol such reactions.Among other functions, enzymes bring together amino acids, com-bine them, and produce proteins. But enzymes are themselves proteins. Producing proteinsin the absence of proteins is biologically impossible, at least in today’s world.

But perhaps in the ancient world this was not so. Speculators offer us several scenarios:

1. When ocean tidal pools evaporate, salts and other impurities left behind becomehighly concentrated. In ancient oceans, evaporation would have concentrated theamino acids, making it much more likely that they would combine and form proteins.

2. Bubbles are common in today’s oceans and would have also been common in anocean of carbon compounds. Powerful electrostatic forces inside bubbles would at-tract amino acids, pulling them into close proximity where again they might interact.Furthermore, when bubbles burst, they spew contents into the atmosphere whereother important chemical reactions could occur.

3. It is also possible that iron pyrite crystals, also known as “fool’s gold,” and clay crystals,both common on Earth’s surface, could similarly attract and concentrate amino acids.

Over perhaps hundreds of millions of years, any, all, or similar processes could havetransformed an ocean of simple carbon compounds into an ocean of more complicatedorganic compounds—proteins, carbohydrates, nucleotides, and phospholipids.

Even in this early ocean, a kind of natural selection would tend to favor certainmolecules over others. Some of these compounds would have been more stable thanothers and would tend to persist. Less stable compounds would tend to disintegrate.Over time, the variety of chemical compounds in the ancient ocean would decrease asmore stable compounds evolved and persisted.

The next step in the evolution of life depended on a peculiar property of some ofthose chemical compounds:The phospholipids.As has been mentioned previously (seeSection 4-4), one end of these rather large molecules is attracted to water, while theother end is repulsed by it. If you swirl a bunch of phospholipids in water, they arrangethemselves into tiny bubbles covered with a two-molecule thick skin of phospholipidswhose water-seeking ends point out and whose water-avoiding ends point in.As we haveseen, today’s cells are surrounded with a cell membrane two molecules thick composedlargely of phospholipids. Tiny bubbles in an ancient ocean are certainly not yet cells.But their outer coverings begin to resemble cell membranes.

Inside ancient bubbles, chemicals were concentrated. The contents of one bubblewould be considerably different from the contents of another.Their existence was tran-sitory; bubbles exist for only a few moments, then burst. Some, by pure chance, mightcontain proteins and other chemicals that would stabilize the phospholipid membranes.These bubbles would tend to persist, again, favored by natural selection. Some, by purechance, might contain proteins that could break down complicated chemicals into simpler


ones, thereby releasing energy. Bubbles in ancient seas might fuse together, mixing con-tents, mixing capabilities. Over time, the more stable and complicated of these fortuitouscombinations of chemical bubbles would persist and thrive. Eventually, they would reacha level of complexity that could be called protocells (Figure 9-3).

Protocells are not yet living organisms. One crucial characteristic of life is stilllacking—the ability to reproduce. For years, scientists assumed that cells could notreproduce without DNA. In today’s cells, DNA is the hereditary material passed fromone cell generation to another. It controls the production of proteins through a muchsimpler intermediary: RNA. DNA is one of the most complicated chemical compoundsknown to science. How might it have evolved?

There is a more basic question: Is DNA absolutely essential? Certain viruses, calledretroviruses, may provide a clue. Retroviruses contain no DNA. Their “hereditary ma-terial” is only RNA. Once inside a host cell, each viral particle contains sufficient RNAto synthesize a new generation of viral particles. Could RNA have done the same inprotocells? In 1993, scientists at the Scripps Institute in La Jolla, California, chancedupon a small molecule of synthetic RNA with remarkable properties.Within an hour ofits synthesis, it began to make copies of itself, and the copies made more copies.Then thecopies began to change—evolve, if you will—acquiring new chemical characteristics.

Is the molecule alive? Not yet, because its existence depends on a steady supply ofpreformed proteins. But in an ancient ocean filled with organic compounds, protocellscontaining RNA with similar properties, plus RNA that could synthesize enzymes ca-pable of breaking down other organic compounds, plus RNA that could synthesize en-zymes capable of building and maintaining cell membranes, might well qualify as the firstcells. Later, DNA would evolve as a method of conveniently and safely storing the vitalchemical information contained in the RNA of the cell.

Is this how life and the first cells evolved on Earth? Possibly, or in some processsimilar to this. Dozens of laboratories around the world are conducting experiments thatare testing hypotheses that may someday synthesize life. It is an active area of research.

Spontaneous generation of amino acids,simple carbohydrates, and lipid precursors

Formation ofself-replicating

RNA

Evolution of DNA

Protocells

Living cells

Evolution of DNA -- RNA -- Enzymes

Formation ofproteins

Formation oflipid bubbles

Figure 9-3 ■ Key events in the chemicalevolution of life.

9-2 What Were the Major Milestones in Earth’s Evolving Biodiversity? 247

Synthesizing Cells What is the current state of attempts to synthesize cells in a laboratory?If scientists are successful in synthesizing “life in a test tube,” what will be the moral, legal, andphilosophical implications?

Piecing It Together

Here’s what we learned:

1. Earth’s early atmosphere was considerably different from today’s, containing muchmore carbon dioxide and nearly no free oxygen.

2. Experiments conducted since the early 1950s have confirmed that simple carbon-based compounds, including amino acids, will spontaneously form under conditionstypical of Earth’s early atmosphere.Astronomers commonly observe carbon-basedcompounds throughout the universe.

3. Over time, naturally occurring chemical processes combined simple carbon-basedcompounds into more complex ones, including proteins, carbohydrates, lipids, phos-pholipids, and nucleotides (RNA and DNA) leading to protocells.

9-2 What Were the Major Milestones in Earth’s Evolving Biodiversity?

However it may have happened, life appeared and changed Earth forever. Roughly4 billion years ago, a process started that transformed a lifeless, albeit wet planet intoa biosphere teeming with life. From simple protocells, millions of species evolved.Each of these is a crowning success story; the end product of a detailed, long evolu-tionary process. Even extinct species were, for a time, successful. To completelyunderstand Earth’s biodiversity, we would have to know each species’ story, past andpresent, which is well beyond the scope of this or any other book. However, we canidentify certain key milestones—special characteristics, the acquisition of which openedfloodgates of possibilities, resulting in numerous new species. In this section, we willexamine those milestones.

A word of caution: There is a tendency to view organisms appearing later in theprocess as somehow superior to those appearing earlier. Indeed, biologists often referto the relatively late appearing flowering plants and backboned animals as more“advanced” than the “primitive,” much older bacteria. Certainly, these “advanced”organisms tend to be more complex than the more “primitive” ones. Advanced organ-isms tend to have more moving parts and utilize and control more complex processes.Outside of biology, we assume ourselves to be preeminent, superior to all other organ-isms. Be careful. Remember that from a biological standpoint, complexity is not success,nor is intelligence or consciousness, although those traits are certainly important to us.Rather, success involves surviving and acquiring enough energy and nutrients to re-produce and pass useful characteristics on to offspring. In those terms, it is hard to arguewith the success of certain microbes that have survived apparently unchanged for bil-lions of years. Let us agree that in this discussion,“advanced” means nothing more than“appeared late in the process.”

Exploration


The First Cells Evolved into the Different Cell Types We See TodayThe first cells would have been totally dependent on the ocean’s preformed carbon com-pounds for both energy and nutrients. Initially, they had no predators—nothing to controltheir populations. What probably happened next is easy enough to duplicate in thelaboratory. Introduce a few bacteria into a beaker of nutrient broth and what happens?The numbers of bacteria soar until nutrients are exhausted, then the population crash-es. In the ancient ocean, the first cells would have done the same.They would have beentotally dependent on the ocean’s nutrients for energy. It would have taken time, ofcourse—the ocean is a huge beaker—but eventually, the first cells would exhaust theancient ocean of its organic soup.

Furthermore, the very existence of these heterotrophic cells, cells incapable of pro-ducing their own food, would make the spontaneous generation of new organic com-pounds impossible. Any simple carbon compound floating around in the environmentwould quickly be gobbled up by a living cell.

With increasing numbers of cells and decreasing nutrients in the early ocean, earlylife faced its first crucial test. Any cell at this time with the capacity to trap and storeenergy would have a tremendous advantage. One process early cells seized upon was totrap the energy of sunlight and store it in the relatively simple chemicals they produced.Chemicals called pigments absorb light energy (discussed more fully in Chapter 10);those related to chlorophyll may have been available early on. Cells that produce chem-icals that store energy are called autotrophs, and their numbers soared. Similar cells,called chemautotrophs, also evolved the ability to store energy contained in certaininorganic chemicals found in or near the ocean.

Autotrophs proliferated as the carbon-based soup played out. Heterotrophs had toswitch from dependence on organic soup to dependence on organic autotrophs; that is, theybecame predators, scavengers, and decomposers. Forevermore, heterotrophs would limitthe numbers of autotrophs, while autotrophs, by their presence or absence, would regu-late the number of heterotrophs.The capacity for balanced ecosystems had also evolved.

These are the organisms that left fossils 3.5 billion years ago in the rocks of westernAustralia (Figure 9-4).A billion years later, some autotrophs evolved methods that im-proved their ability to trap and store the energy of sunlight, and modern photosynthesisevolved.These new autotrophs were so much more efficient at storing energy that theirnumbers must have increased. One of the by-products of photosynthesis is free oxygen,

Figure 9-4 ■ These fossils from western Australia are some of the most ancient multicellularorganisms ever found. (a) Mawsonia springgi, (b) Dickinsonia costata.

(a) (b)


which most organisms of the ancient world found intolerable. In the oceans, processesthat built first simple and then complex organic compounds removed much of the carbondioxide from the Earth’s atmosphere. Now latter-day autotrophs removed most of therest and replaced it with free oxygen.Their excess oxygen forever changed the chemicalnature of the Earth’s atmosphere. The atmosphere we know today had evolved.

Cellular life could now evolve into what we know it is today. Organic soups and pro-tocells were gone forever, incompatible with the new conditions of the ocean andatmosphere. Cells, perhaps identical with the first oxygen-intolerant heterotrophs, are stillpresent as the Archaea.They are confined to deep muds, inside carcasses, within intestinaltracts of more complicated organisms, and in other places that are free of oxygen. Chemau-totrophs are with us still, found near volcanic vents deep in the ocean, in hot sulfur springs,and similar environments. Today’s blue–green algae or cyanobacteria must at least besimilar to the early prokaryotic autotrophs.They still trap the energy of the sun and makeit available to other organisms.

Simple Cells Evolved into More Complex CellsOther improvements in cellular efficiency also evolved. The first cells were necessarilysimple in structure with few internal parts, that is, prokaryotes. They are unquestion-ably still successful. Some thrive in hot springs and acid baths that are totally intolera-ble to other organisms. Others have lain dormant but viable for hundreds, indeed,thousands, of years.

But a lack of internal parts apparently imposes limitations on the complexity oftheir internal chemistry—the efficiency with which they process nutrients, produceproducts, and handle energy. Somewhere between 2 and 1.5 billion years ago, a new kindof cell appears in the fossil record. These cells, the eukaryotes, had membranes on theinside that produced complex systems of parts and pockets in which particular kinds ofchemical reactions could be isolated from others. (More information about prokaryoticand eukaryotic cells can be found in Chapter 4 and on the BioInquiry web site.)

Two processes seem to have been involved in the transition from prokaryotic to eu-karyotic cells. In-pocketing and refining of the plasma membrane resulted in a numberof membrane-bound organelles.Thus, eukaryotes obtained their nuclei, vacuoles, Golgicomplexes, endoplasmic recticula, and so on.

Apparently, eukaryotes obtained mitochondria and chloroplasts from an entirelydifferent process, termed endosymbiosis. According to endosymbiotic theory, precursormitochondria were originally prokaryotes that evolved means to efficiently obtain energyfrom glucose. Primitive eukaryotes, lacking this ability, would benefit immensely frompartnering with them. Each would benefit by associating with the other. Such intimateassociations, symbioses, are fairly common among organisms. In time, the associationbecame so intimate that mitochondria came to resemble just another organelle in euka-ryotic cells. In a similar way, the first eukaryotic plant cells obtained their chloroplasts.

Extreme Intimacy What evidence supports endosymbiosis? Are there other examples ofsuch partnerships involving other organisms?

Single-Celled Organisms Evolved into Multicelled OrganismsSome time around 1 billion years ago, multicellular organisms appeared. Groups of cellsliving together were nothing new; some of the most primitive prokaryotes formed andstill form colonies of single cells that simply fail to separate after cell division.Why aren’tthese colonies considered multicellular organisms? Each cell in the colony is capable ofan independent existence. If separated from the others, each has the potential of be-coming a new colony. In a colony, each cell is connected to others, but depends on themfor nothing.

Exploration


Multicelluarity evolved whenever some colonial cells specialized—for example, con-centrating on movement, food digestion, or reproduction—and other cells became de-pendent on them for those functions. Volvox (Figure 11-1) is an example of perhaps thesimplest multicellular organism. Cells on the inside, daughter cells, are the organism’sonly way of reproducing. Cells on the outside, forming the hollow ball, are responsiblefor all other functions.

In more advanced multicellular organisms, cells specializing in a particular functionare known as tissues. There is considerable variation among organisms in the numberand complexity of tissues. Simple animals—fungi and algae—have relatively few. In gen-eral, plants have fewer tissues than animals. It takes more than 200 different tissues toform a mammal or bird.

So, a great deal of evolution occurred among the multicellular eukaryotes. Today,some are autotrophs (plants) and some are heterotrophs (animals and fungi). All arecomposed of cells with similar parts and capabilities.

The Evolution of Animals Involved Several MilestonesGenerally speaking, animals—multicellular, eukaryotic heterotrophs—are the most ad-vanced organisms. Their evolution is marked by several unique, significant milestones,which are summarized in Figure 9-5.

One of these milestones is the presence or absence of tissues. The most primitiveanimals, sponges, lack true tissues. While they are composed of several different typesof cells, each performing specific tasks on which other cell types depend, their cells lacksufficient integration to be considered true tissues. If a sponge is pressed through a finesieve, so that the result is little more than a mass of dissociated cells, the cells will, giventime, reorganize into a new sponge. In addition, they have remarkable abilities to re-generate. Cells specialized for one function can, if necessary, specialize into another typeof cell. It seems best to think of sponges as an intermediate form on the path from colo-nial to multicellular organisms. All other animals have true tissues.

A second milestone involved overall body form or shape—its symmetry. Jellyfish andrelated animals have definite belly and back surfaces, but no heads or tails nor right or leftsides. Their digestive systems have a single opening generally found at the center of thebelly surface. Other body parts, including muscles and tentacles, radiate out from a centralaxis passing through the mouth.These animals have radial symmetry.Animals with radialsymmetry are aquatic and tend to either stay in one place throughout most of their livesor move vertically. Jellyfish, for example, tend only to move up in the water column andglide down.To move horizontally, they are more or less at the mercy of currents.

More advanced animals show bilateral symmetry. Definite left and right are on ei-ther side of an axis joining head to tail. In animals with bilateral symmetry, sense or-gans tend to be concentrated at the head end. Generally speaking, animals with bilateralsymmetry are capable of more complex movements.

Starfish and their relatives appear to be exceptional. These are obviously not prim-itive animals, yet they have radial symmetry and limited mobility. In starfish, legs radiateout from a central axis. But their embryos and larvae are bilaterally symmetrical, sug-gesting that they evolved from bilateral ancestors.Their radial symmetry, as adults, is be-lieved to be a reversion to a primitive characteristic reflecting where and how they live.

Embryos develop layers of cells called germ layers, from which their organs develop.The embryos of primitive animals have two such germ layers, an ectodermis, or outerskin, and an endodermis, or inner skin. More advanced animals have three germ layers.Between their ectodermis—the origin of their outer skin—and endodermis—the origin oftheir digestive system—is a mesoderm—the origin of their muscles and associated organs.

Germ layers gave rise to another milestone in animal evolution, the origin of internalbody cavities or coeloms. (We have three such cavities:The abdominal cavity, the thoracicor chest cavity, and the pericardial cavity around the heart.) Primitive animals, such as


Ancestor to Animals

TISSUES

SYMMETRY

GERM LAYERS

BODY CAVITIES

EMBRYONIC CELLCOMMITMENT

SKELETON

BACKBONE

No true tissues True tissues

Radial2-germ layers

Bilateral3-germ layers

No coelom

Pseudocoelom True coelom

Late Early

No hard skeleton Exoskeleton ExoskeletonEndoskeleton

No backbone Backbone

roundworms

sponges

flatworms

segmentedworms shellfish arthropods

backbonedanimals

spiny-skinnedanimals

jellyfish

Figure 9-5 ■ Milestones in the evolution of animals.

flatworms,have no such cavities; they are acoelomates. In other relatively primitive animals,such as the roundworms, the cavity develops between the endoderm and mesoderm; theseare pseudocoelomates. Advanced animals have a cavity completely surrounded bymesoderm; these are coelomates. When filled with fluid, these cavities can serve severalfunctions. Nutrients and wastes can circulate within the fluids. Under pressure, the fluidsform a primitive hydrostatic skeleton, giving the body rigidity and assisting in movement.


Where and how tissues develop is another milestone in animal evolution. All ani-mals start life as a single, undifferentiated cell: The fertilized egg or zygote. By the timethe animal is born, it is composed of thousands to trillions of cells, all derived from theoriginal fertilized egg. Furthermore, the resultant cells specialize into tissues and organs.Animals vary in the timing of cell specialization. In primitive bilateral animals, cellscommit to specialization early.When the embryo of a segmented worm is in the four-cellstage and if one cell is removed, the embryo is incapable of developing into a completeindividual; the individual resulting from the remaining three cells lacks key organs. Inmore advanced animals, the commitment to specialize occurs much later.A cell separatedfrom a four-cell mammalian embryo will grow into a complete new individual—a cloneof what develops from the other three cells.

Another milestone occurring within the coelomates involves the fate of the blasto-pore. As embryonic cells divide, they organize. First to develop is a hollow ball of cells,the blastocyst. As further cell divisions occur, the hollow ball indents, first slightly, buteventually into a two-layered hollow ball, the gastrula. At this stage, all that remains ofthe indentation is a small opening, the blastopore. Now, in segmented worms, shellfish,and arthropods, it eventually becomes the mouth; in spiny-skinned animals and back-boned animals, it becomes the anus.

Finally, among the coelomates, further differentiation seems to have involved thetype of skeleton. Segmented worms retained the ancestral hydrostatic skeleton. Shell-fish and arthropods (jointed-legged animals such as insects) evolved exoskeletons,which means they have skeletons on the outside of their bodies. Such skeletons givemaximum protection to soft tissues and provide nearly unlimited attachment sites formuscles. On the debit side, exoskeletons are heavy and unwieldy; they impose limits tooverall size. The most advanced animals—the spiny-skinned animals (such as starfish)and the back-boned animals—evolved endoskeletons, that is, internal skeletons.Theseprovide adequate protection to critical soft tissues and muscle attachment sites whilecutting down on weight.

And so a 4-billion-year odyssey has seen life on Earth pass from protocells to theunimaginable diversity we see today. Millions of species live nearly everywhere on,below, and above Earth’s surface. Next let’s see how biologists classify and keep trackof so many species.

Piecing It Together

As biodiversity evolved on Earth, life acquired special characteristics, the acquisitionof which stimulated proliferations of new species. These included the following:

1. Earth’s first organisms were single-celled heterotrophs. Lacking the ability to maketheir own food, they first consumed naturally occurring carbon-based compounds.Later, as these compounds became exhausted, they evolved into single-celled andmulticelled organisms that exploited other organisms for food:Today’s decomposers,scavengers, and predators.

2. Autotrophs evolved from heterotrophs. They produce much of their own food byutilizing the energy of chemicals or trapping the energy of sunlight and are today’salgae and plants.

3. Eukaryotic cells evolved from prokaryotic cells. Two processes were involved:(1) in-pocketing and specialization of cell membranes and (2) endosymbiosis.Today’s prokaryotes are the bacteria and blue–green algae. Today’s eukaryotesare all other organisms.

9-3 How Do Biologists Keep Track of So Many Species? 253

4. From colonial, single-celled prokaryotes, multicelled organisms evolved into today’sfungi, plants, and animals.

5. Animals evolved from single-celled heterotrophs. Their further evolution involvedseveral special milestones. These includeda. The presence or absence of tissues.b. The type of body symmetry, radial or bilateral.c. The number of embryonic germ layers, either two or three.d. The presence or absence of internal body cavities (coeloms). Bilateral animals

fall into three categories: Those with no such cavities (acoelomates), those witha cavity between the mesoderm and endoderm (pseudocoelomates), and thosewhose coeloms are surrounded with mesoderm (coelomates).

e. The embryonic timing of cell specialization among the coelomates. The cells ofprimitive coelomates commit early to cell specialization, while the more advancedcoelomates commit later in embryonic development.

f. The location and the way tissues develop. In primitive coelomates, the blastoporebecomes the mouth. In advanced coelomates, it becomes the anus.

g. The presence or absence of skeleton, as well as the type of skeleton among themore advanced coelomates. Segmented worms have only a hydrostatic skeleton.Shellfish and arthropods have skeletons on the outside of their bodies (ex-oskeletons). Spiny-skinned animals and those with backbones have internal skele-tons (endoskeletons).

9-3 How Do Biologists Keep Track of So Many Species?

Supermarkets face problems similar to biologists—namely, how to organize thousandsof brand names and food types so that they can be easily found by shoppers and easilystocked, maintained, and inventoried. Suppose you’re shopping for a can of peas. Thestore might arrange food in alphabetical order. On a shelf in the middle of the store youwould find “Peas, canned,”“Peas, dried,” and “Peas, fresh.” No problem for the shopper,but next to impossible for the grocer.

Instead, supermarkets are arranged in a series of sections,“Dairy products,”“Meat,”“Produce,” “Package goods,” “Gourmet foods,” etc. To find a can of peas, go first to“Package goods,” then find “Canned goods,” then “Canned vegetables,” and finally“Canned peas.” Note that the organizational system is based on similarities and differ-ences.All package goods share certain characteristics that distinguish them from produce.Note, too, the system is hierarchical. “Chicken” is a subset of “Meats” and is itself sub-divided into “Whole chickens” and “Chicken parts,” which are then subdivided into“Breasts,” “Drumsticks,” and so on. Finally, note that the system is somewhat arbitrary.Canned garbanzo beans may be considered canned vegetables in one store and agourmet item in another (Figure 9-6).

Whereas grocers deal with a few thousand items, biologists deal with roughly a mil-lion and a half species and anticipate dealing with many more. Their system of organi-zation is also based on comparisons, hierarchical groupings, and is somewhat arbitrary.Not surprisingly, biology’s system of classification took several hundred years to develop.

Prior to Darwin, Classification Was Concerned with Describing “Natural Order”As with so many other areas of biology, taxonomy can trace its beginnings to Aristotle.He arranged objects, including animals, into groups through a series of “either–or”comparisons. Either an object was living or nonliving, animal or plant,“blooded” or “non-blooded,” with feet or without, many footed or four footed, and so on.This resulted in a


listing of animals into what was perceived to be a natural order leading progressivelyfrom the simplest animals to the most complex. His principal student,Theophrastus, usedsimilar either–or groupings to classify plants as trees or shrubs, subshrubs or herbs.Theirsystem of classification, such as it was, persisted with little change for nearly 1,500 years.

The 14th century saw a vast increase in exploration, as mariners, particularly fromEurope, plied the world’s oceans. Many expeditions included naturalists, who describedand collected specimens and brought them back to Europe. For several hundred years,museum collections grew and as they did, so did the need to organize, classify, and namethe organisms.

Throughout these early days of classification, the dominant philosophical view wasthat species were fixed, unchanging, and could be arranged in some sort of natural order.The job of the taxonomist was to describe species as accurately as possible. This wouldserve, first, to assist in accurate identifications and then, perhaps more importantly, toreveal the structure of life’s natural order. Each species was seen as a separate type oflife, perfectly conceived. Species descriptions sought to list each species’ idealized char-acteristics, which became the archetype for that species. The perfect or archetypal rosewas seen to have one set of characteristics, while the archetypal dog another. Individualvariations were largely ignored.

The archetypes that shared a particular set of characteristics could be arranged to-gether in groups.At first there was little agreement as to which characteristics should bechosen for grouping or how the groups should be arranged. By the beginning of the 18thcentury there were dozens of separate classification systems; some focused on plants whileothers focused on particular groups of animals. Organizationally, they shared little in com-mon and were largely incompatible. Biological classification was on the verge of chaos.

Into this chaos came the Swedish botanist Karl von Linné, who lived from 1707 to1778. He is one of biology’s most enigmatic and complicated characters. He wasinterested in numerology, especially taken by the numbers 5, 12, and 365. He was highlyreligious and, later in life, became a mystic. He had great literary powers and enjoyedgreat fame during his life. He traveled widely, but spoke only Swedish and Latin. Linnéwas also a pragmatist who wrote meticulously detailed descriptions of plants and animals.His obsession with classification knew few bounds: Not only did he classify plants, hedeveloped an elaborate classification of botanists (plant physiologists, “botanophiles,”collectors, and so on).

Linné’s contributions were great. By the strength of his personality and sheer volumeof work, he brought simplicity and consensus to the chaos of the field and, in the process,described over 8,000 species of plants and animals (Figure 9-7).To each he assigned two

Figure 9-6 ■ Modern supermarkets areorganized so that it is relatively easy tofind a particular food item among racksand stacks of thousands of similar items.The classification system used by biologistscan bring similar order to and keep trackof millions of species.


Figure 9-7 ■ Carolus Linnaeus named over8,000 organisms, including (a) the domesticdog, which he named Canis familiarus, (b)the club moss, which he named Lycopodiumsalago, and (c) the red fox, which he namedVulpes vulpes.

Latinized names, unique to each species. (Latinized names were so important to himthat he changed his own name to Carolus Linnaeus.) The first name was the genus name;closely related forms could share this name and thus be grouped together. The secondname was the species name, which was not shared by closely related forms. For example,in the genus Canis, he assigned to the domestic dog the scientific name Canis familiarusand to the wolf Canis lupus. This tradition of binomial nomenclature continues in thenaming of species today.

To deal with so many species, Linnaeus saw the need to organize them into highertaxonomic categories. Related genera (plural of genus) were combined into categorieshe called Orders, which were similarly combined into Classes. The highest taxonomiccategories were Kingdoms—Plant or Animal. He did this as a matter of necessity. “Anorder,” he wrote, “is a subdivision of classes needed to avoid placing together moregenera than the mind can easily follow.” Whereas he felt his descriptions of speciesand genera represented “natural order,” he saw orders and classes to be largely artificial.

The Linnaean system of classification was widely accepted by mainstream biology.Buffon, a contemporary of Linnaeus, Lamarck, one of Buffon’s students, and Cuvier(refer back to Chapter 2 for more extensive discussions of some of their other contri-butions to biology) accepted and refined the Linnaean system, concentrating on ani-mals. Others extended Linnaeus’ work with plants. Thus, the task of describing Earth’sperfect species and life’s natural order continued.

After Darwin and Mendel, Emphasis Shifted to the Ancestry of SpeciesDarwin’s theory of evolution through natural selection stimulated a complete reevalua-tion of the meaning of biological classification. Prior to Darwin, classifiers saw them-selves describing fixed, immutable, perfect entities.They placed no special significance onthe fact that species shared characteristics. That species did was simply a convenience

(a)

(b)

(c)

Classification:Determine criteriato classify aspecies.


that allowed them to be grouped. Darwin not only viewed species differently, he providedan explanation of why certain characteristics were shared. Rather than perfectlyimmutable entities that could be arranged into a “natural order,” he saw species as con-stantly, albeit slowly, changing.The fact that they shared characteristics was to be expected,since all species, at some time, shared common ancestors. Species that shared the mostcharacteristics were the most closely related.Thus, dogs are closely related to wolves, andneither is too far removed from their common ancestor. Dogs and cats are more distantlyrelated; their common ancestor is further back in evolutionary history.

Darwin’s theory had little effect on the everyday work of the taxonomist. Thearchetype was now replaced with type specimen, which was either the first specimen col-lected or a representative specimen of a given species.A detailed description of the typespecimen defined each species’ essential characteristics against which subsequent spec-imens could be compared. Individual differences between specimens were to be expectedafter Darwin’s theory became known.They were the expected result of natural selection.

Mendelian and population genetics also affected biological classification by stress-ing the importance of populations. Species were no longer seen as “types of life,” butrather as sets of individuals that could interbreed. The job of the taxonomists becameto describe those characteristics that differentiated one distinct population of a speciesfrom another. Type specimens are still collected and described, but any given charac-teristic within a population was expected to range through a predictable set of values.For example, the weight of an adult male wolf is not expected to match some idealizedvalue, but to range between 20 and 80 kilograms (44 to 175 pounds).

Today, Modern Tools Are Yielding Direct Evidence of Evolutionary Relationships between SpeciesThe primary objective of modern systematics is to describe the evolutionary relationshipsbetween species. In Chapter 2, we defined species as “a group of organisms that caninterbreed in nature.” Determining whether or not individuals interbreed is often difficultand sometimes impossible under natural conditions. Traditionally, taxonomists definedspecies by comparing their structures, forms, and other relatively easily observed charac-teristics.They assumed that if individuals are different they cannot or will not interbreed,but even this is not as straightforward as it might sound. First, natural selection is not aneven-handed process. Different populations change at different rates. At what point twopopulations have differentiated enough to be considered separate species is sometimes adifficult judgment call. Natural selection does not make life easy for the taxonomist.

Inferring evolutionary relationships between species is often more difficult still.Sometimes, distantly related species evolve similar characteristics when faced with similarenvironmental challenges (convergent evolution was discussed in Chapter 2). Comparedolphins (mammals), sharks (primitive fish) and ichthyosaurs (extinct marine reptiles)(Compare Figures 2-14 and 9-8). If they share so many characteristics, why aren’t theymore closely related?

Under other circumstances, closely related species may differentiate. (Divergentevolution was also discussed in Chapter 2.) Hummingbirds seem to have little in com-mon with ostriches. What makes us think they are related?

In spite of these and other difficulties, taxonomists and systematists regularly identifynew species and infer relationships between them. Given the complications, it is perhapsnot surprising that adjustments are sometimes necessary. Not too many years ago, snowgeese (Chen hyperborea) and blue geese (Chen caerulescens) were thought to be sepa-rate species and, although inbreeding was known to occur, the offspring were assumedto be sterile hybrids. When this was seen not to be so, snow geese and blue geese werecombined into one species, Chen caerulescens (Figure 2-10a and b). Blue forms and whiteforms are now seen as color phases of the species, that is, individuals with differentappearances. Red foxes of Europe (Vulpes vulpes) were once thought to be separate


from red foxes of North America (Vulpes fulva). How could they possibly interbreed,separated by an ocean? Recent historic evidence suggests red foxes were released inthe southeastern United States from Europe around 1750.Today, we recognize only onespecies of red fox on both continents, Vulpes vulpes. Determining relationships throughcomparison of structures and form is seldom easy.

Recently, new technologies in molecular biochemistry are making systematics morestraightforward. Relationships between species can be worked out by comparing theirproteins, RNAs, and DNAs. Early efforts compared various species’ cytochrome c, anenzyme present in most cells. First, samples of cytochrome c were isolated from cell sam-ples of a variety of species. Next, the order of their amino acids was determined. Mutationschange the order of amino acids in proteins and are assumed to occur at relatively con-stant rates. Closely related species have so recently separated from their common ancestorthat few mutations should have occurred and the structure of their cytochrome c shouldbe similar.Through similar reasoning, more distantly related species should show greatervariations in their cytochrome c.

Similar analyses compare species’ mitochondrial DNA (mtDNA), ribosomal RNA(rRNA), and nuclear DNA. For the most part, these techniques have yielded fewsurprises. Traditional taxonomists and systematists did their work carefully, and therelationships they worked out have passed yet another test of validity. These chemi-cal tests are most valuable in settling controversies. For example, evolutionists havelong contended that the first land vertebrates, early amphibians that are related totoday’s salamanders and frogs, evolved from bony fish. But which group of fishes(Figure 9-9)? To answer, we might rephrase the question, “Which group of fish sharethe most characteristics with primitive four-legged animals?” Two distantly relatedcandidates emerge: Lungfish have lungs, which the first terrestrial animals surelyrequired, so perhaps they were the ancestors to early amphibians. But the lobe-finnedfish, of which the Coelacanth is an example, has fins with a tetrapod-type skeleton.Over the years, most specialists have favored lobe-finned fish as the amphibian ance-stor, but recent comparison of rRNAs suggests otherwise.The sequence of nucleotidesin lungfish more closely resembles that in amphibians than in lobe-finned fish. Isthe question settled? Not yet. Remember, biologists are always skeptical. Most wouldlike to see more comparisons of protein and nuclear DNA structure. But the scale ofprobability tips a bit toward lungfish.

Classifying so many different organisms is a daunting and sometimes esoteric task.Exceptions abound and make life for the taxonomist interesting. An ancient Chineseproverb has it that naming things is the first step to wisdom. Try being a taxonomist.As you encounter organisms day to day try to classify them. Look for their similaritiesand differences. It’s not always easy, but it can be rewarding.

Figure 9-8 ■ Although ichthyosaursresemble dolphins and sharks (see Figure2-14), they are not considered to be closelyrelated. Ichthyosaurs had no mammaryglands and were therefore not mammals.Their skeletons were made of bone, notcartilage, as are sharks and their relatives.Because of the structure of their skeletons,ichthyosaurs are classified as reptiles.


Piecing It Together

1. The purposes of biological classification are threefold:

◆ To assist in species identification

◆ To assign formal, consistent scientific names to species

◆ To describe ancestral relationships between species

Early amphibian

? ?

Lobe-finned fish Lungfish

Figure 9-9 ■ Who were the ancestors of early amphibians? Because of similarities in theirskeletons, it appears that amphibians evolved from bony fish. But from which group of fish?The pattern of bones in the amphibian’s limbs closely resembles the bones in the lobe-finnedfish’s fins. Therefore, they were thought to be the ancestors to amphibians. But recent studiessuggest otherwise. Comparisons of DNA of these three animal groups suggests that the morelikely ancestors of early amphibians were lungfish.

9-4 How Does the System Work? 259

2. Traditionally, species have been described and relationships proposed through de-tailed descriptions, stressing structure and form. Recently developed biochemicaltechniques allow comparisons of amino acid sequences in proteins and nucleotidesequences of mtDNA, rRNA, and nuclear DNA.

3. Biology’s classification system is both hierarchical, based on groups within groups,and comparative, based on the characteristics groups share. Groups and shared char-acteristics are more than conveniences; often they show evolutionary relatedness.

9-4 How Does the System Work?

Stated simply, the work of the taxonomist is to place organisms into a series of hierarchicalgroups called taxa (plural of taxons). The broadest taxonomic groups contain the mostorganisms and are themselves broken up into less inclusive groups, which are subdividedstill further until the level of individual species is reached.All organisms within a particulartaxon share certain characteristics. Until recently, the choice of which characteristics touse for taxonomic comparisons was somewhat arbitrary and often involved analyses ofhomologous structures (discussed in Chapter 2).As new species evolve they inherit andmodify the characteristics of their ancestors. As we shall see, interpreting relationshipsbetween organisms, the work of the systematist, is far from arbitrary.

At First, There Were Thought to Be Only Two Kingdoms; Now There Are at Least FiveFrom the time of Aristotle to the middle of the 20th century, biological classifiers first di-vided organisms into either of two kingdoms: Every organism was either plant or animal.These were the largest of the taxonomic groups.

In order to be classified a plant, an organism had to possess certain characteristics.Plants tend to be sessile; that is, they don’t move around much. Structurally, they are madeof stems, roots, and leaves that are generally green. By the middle of the 19th century, itwas known that plant cells are surrounded by thick walls. Later, the significance of greenwas realized: Plants are photosynthetic and produce their own food.Animals possess othercharacteristics.They are generally more responsive; that is, they move around more thanplants. Their cells have no walls, and they don’t produce their own food. The first step ofthe taxonomists—placing organisms into kingdoms—seemed straightforward and easy.

There were some exceptions. What about fungi? They are generally sessile, withthick-walled cells, but they are not photosynthetic. To be grouped together, organismsdon’t need to possess all characteristics of a given group; where necessary, shared char-acteristics were prioritized. In the case of fungi, thickness of cell walls was thought to bemore diagnostic than mode of nutrition.The predominance of evidence suggested fungiwere plants. Bacteria, too, were included as plants.

Other exceptions were not so easily dealt with. A microscopic, one-celled organismnamed Euglena was particularly worrisome. (See Figure 9-17b.) It has a long flagella andno cell wall. During summer, Euglena has well-defined chloroplasts, gathers at the surfaceof ponds, and produces its own food. But with light less available in winter, particularlyunder ice,Euglena loses its chloroplasts,descends into bottom muds,and moves from placeto place decomposing dead material. In summer,Euglena is clearly a plant; in winter, clearlyan animal.Which is it, really? Not even prioritized characteristics help much here.

In the late 1960s, a radical solution gained acceptance: Create more kingdoms toaccommodate the exceptions. Kingdom Protista would include not only Euglena, butalso all similar organisms.All protists have eukaryotic cells, that is, cells with organelles.

History of Classification:Observe how species are classified.

Kingdoms: See what criteriadefine a kingdom.

Fungi: Fungiare eukaryotic.

Protista: Protists are unicelluar and eukaryotic.

Exploration


Some are photosynthetic, some are not. Some have thick cell walls, some do not. King-dom Monera would accommodate bacteria, single-celled prokaryotes. (If you’ve for-gotten the characteristics of prokaryotes, review Section 4-2.) A third new kingdomincluded fungi. Biological classification entered into a five-kingdom phase—Monera,Protista, Fungi, Plantae, and Animalia (Figure 9-10).

Still, there were exceptions.What should be done with green algae? Most are single-celled, but many are not. Indeed, multicellularity evolved in this group, although it mayhave evolved independently in the seaweeds known as brown algae.Today, there are nosingle-celled brown algae, but fossilized forms are known. Obviously related to the brownalgae are the red algae, another kind of seaweed. Are these groups protists or plants?There is not much agreement among biologists. Some consider them primitive plantswhile others—and we will follow this convention—lump them with protists.

In Chapter 4,we discussed the possibilities of a third type of cell in addition to prokary-otes and eukaryotes, the Archaea.The existence of a third cell type creates special problemsfor taxonomy.Where do the new cells fit into biological classification? Today, we recognizea new taxonomic category larger than kingdoms, the domain. There are three domains(Figure 9-11).The Domain Archaea includes the newly discovered cell types.The DomainBacteria includes the other members of the old Kingdom Monera. Each of these domainshas one kingdom, namely Archaebacteria within the Domain Archaea and the Eubacteriawithin the Domain Bacteria. Domain Eukarya includes all of the kingdoms and organismscomposed of eukaryotic cells. Notice that within the Eukarya, there are several kingdoms,namely, protista, fungi, plantae, and animalia. Will there be additional kingdoms withinArchaea and Bacteria? Probably. Because of the inherent difficulties in studying them,bacterial classification has lagged far behind that of other organisms. As that situationchanges in the next few years, our understanding of bacterial diversity is bound to expand.

Another Plasmodia What other organisms are called Plasmodia? Why are they called that?Why are they important and how are they being controlled?

Plantae

Animalia

ProtistaFungiMoneraFigure 9-10 ■ How many kingdoms arethere? Formerly, all organisms wereclassified as either Plant or Animal. Thesebecame the first kingdoms. More recently,as the differences between organismsbecame more understood, organisms werereclassified into five kingdoms.

Archaea Bacteria Eukaryaprokaryotes

Other kingdoms being worked out

Archaebacteria Eubacteria

prokaryotes eukaryotes

Plantae Animalia

Protista Fungi

Figure 9-11 ■ The recently proposedaddition of Domains, a taxonomiccategory larger and more inclusive thankingdoms, represents a major change tothe system of classification usedby biologists. The basis of distinctionbetween domains is the structure andchemical composition of cells.

Plantae: Plantae are

multicellular, eukaryotic,

and autotrophic.


A third cell type creates even more problems for systematists. What are the evolu-tionary relationships between the domains? Much more data are needed before thisquestion can be answered with certainty. At least some leading questions can be pro-posed:What does it mean that Archaea share more genes with Eukarya than with Bac-teria? Which domain evolved first?

So, how many kingdoms are there, really? The answer is, as many as we need tohave. Remember that biological classification is a work in progress. Furthermore, it iswork that has only just begun.

Within Each Kingdom, There Are Many Species, Requiring Additional CategoriesGroups within groups and shared characteristics—these are the hallmarks of the modernbiological classification system. Let’s trace a path through this system to see how it works.

All members of the Domain Eukarya have one characteristic in common: Theircells are eukaryotic (Figure 9-12). Further, this characteristic excludes all members ofother domains; that is, no Bacteria or Archaea possess this trait.

(a) (b)

(d)

(e)

Figure 9-12 ■ In spite of their obvious andgreat differences, all of these organisms areplaced in the same domain, Eukarya.Organisms within this domain are composed ofone or more eukarytotic cells, that is, cells withinternal membranes and organelles. (a) Diatom(magnified 9,200 times); (b) fungi; (c)roundworm (magnified 156 times); (d) fairy tern.


Within the Domain Eukarya are literally millions of different species, single-celledand multicelled. Some have cells with thick walls, while others have cells with no walls.These two criteria go a long way to differentiate the eukaryotes into kingdoms. Multicel-lular organisms with cells that lack walls belong in the animal kingdom. Biologists preferthe name Kingdom Animalia, following Linnaeus’ lead in Latinizing taxon names.

Now think of all the different kinds of animals.The variety is still nearly bewildering,but they can be grouped. Each group is a subdivision of the next larger taxon (Figure 9-13).For example, kingdoms are subdivided into groups called phyla. (Botanists prefer to usethe taxon divisions to group plants at this level.) Simple organisms with tentacles such asjellyfish form one phylum. Those with no legs at all, such as worms, form several phyla.Many different kinds of animals have legs; numbers vary from two to dozens. Let’s followthe classification of organisms with backbones; they belong to the Phylum Chordata.

Within the Phylum Chordata, there are many kinds of animals with backbones, still“too many for the mind to grasp,” as Linnaeus might say. More taxonomic categories areneeded, and in the modern system of classification, phyla are subdivided into classes.Phylum Chordata is subdivided into several classes of fish and several classes that, for themost part, live on land. One group of terrestrial, backboned animals are warm-blooded,have hair as a body covering, and possess mammary glands with which females feed young.These are the mammals, Class Mammalia.

Hairy Whales? Given the above, why are whales considered mammals?

Classes are subdivided into orders, of which Class Mammalia has several. One includesall the monkeys, apes, and their relatives—Order Primate.What do you suppose are thecharacteristics that would differentiate primates from all other mammals?

Domains

Species

Genera

Families

Orders

Classes

Phyla/Divisions

Kingdoms

Figure 9-13 ■ The system ofclassification used by biologists is a seriesof groups within groups based onsimilarities and differences.

Exploration

Animalia: Animalia are multicellular,

eukaryotic, and heterotrophic.

Exploration


Orders are subdivided into families.Within the primate order, monkeys can be sep-arated from the so-called great apes. The great apes belong to the Family Hominidae.

This family includes gibbons, orangutans, gorillas, chimpanzees, and humans, each ofwhich constitutes one or more genera, the taxon into which families are divided. Humansbelong to Genus Homo.

In times past there were several human ancestors, today known only from fossils.These are the species included in the Genus Homo.Today, there is only one example ofthis genus, modern humans, Homo sapiens. In Latin this means “self, the wise.”

The complete classification of humans is as follows:

Domain Eukarya (eukaryotic cells)

Kingdom Animalia (multicellular; no cell walls)

Phylum Chordata (backboned animals)

Class Mammalia (warm-blooded, hair, mammary glands)

Order Primate (opposable thumbs)

Family Hominidae (no tails)

Genus Homo (“self”)

Species sapiens (“wise”)

Every species has a similar set of taxa to which it has been assigned, based on its sim-ilarities and differences with other organisms.

In particularly numerous groups, especially insects and certain plants, additionaltaxa are sometimes required.These are formed by using prefixes the “sub” (below) and“supra” (above). Thus, between kingdom and phylum, taxonomists could, if necessary,have subkingdoms and supraphyla. These refinements are available at all taxonomiclevels, wherever needed.

When writing scientific names, biologists use certain conventions. Note that all namesare Latinized. Usually, not all taxa are listed, but genus and species often are (and alwaysshould be in biological writings). Genus is capitalized; species is not. Scientific names areitalicized or underlined to distinguish them from other kinds of terms. In scientific writ-ing, the second reference to an organism within a paragraph or so can be abbreviated.For example, the bacterium Escherichia coli is often referred to as E. coli (but not thefirst time it is referred to in a writing).

An Unlikely Friend: If E. coli are normally found in everyone’s intestine, why do publichealth officials get excited when they turn up in drinking water? And what about the E. coli thattaint hamburgers and cause serious diseases? What is being done to control pathogenic E. coli?

The biological system of classification is extremely useful to biologists. It can ac-commodate any number of species.A newly discovered organism can be fit into the sys-tem by noting what characteristics it shares with other organisms. For example, if it haseukaryotic cells, it belongs in the Domain Eukarya. If it is a multicellular eukaryote, hascells with walls, and is photosynthetic, it is a plant.

The system is somewhat arbitrary in its choice of characteristics used for comparison.Ideally, characteristics are chosen that easily separate groups, irrespective of importanceto the organism. For instance, knobs on the end of antennae separate butterflies frommoths.A characteristic with seemingly no overwhelming importance to the organism isof extreme importance to the taxonomist (Figure 9-14).

The system is also focused on interpretation.Theoretically, taxa indicate evolution-ary relationships. Organisms that share the same genus are quite closely related. Thosewhich share only domains are distantly related. Furthermore, relationships denote evo-lutionary history. At whatever point two organisms stop sharing taxa is the point atwhich they stopped sharing ancestors. Felix onca (jaguar) and Felix concolor (mountainlion) shared a common ancestor a few million years ago. These two cats (Domain Eu-karya) shared a common ancestor with E. coli (Domain Bacteria) billions of years ago.

Organism Identification:Match characteristicswith type of organism.


It is beyond the scope of this book (or, indeed, any other single volume) to reviewall of Earth’s biodiversity. Instead, we will hit some high points. To date, taxonomistshave described and classified roughly 1.5 million species into three domains, six king-doms, and dozens of other taxa. New species are being added to the lists daily.The majorcharacteristics of some major groups are discussed in Appendix C.

Piecing It Together

1. The biological system of classification catalogs, organizes, characterizes, and namesthe millions of organisms that constitute life on Earth. Basically, taxonomists place or-ganisms into various classification categories, called taxa, based on their similarities anddifferences with other organisms. Systematists describe the evolutionary relationshipsbetween organisms and taxa.All organisms are placed in the following categories:

Domain (largest, most inclusive taxon)Kingdom (subdivisions of domains)

Phylum or Division (subdivisions of kingdoms)Class (subdivisions of phyla or divisions)

Order (subdivisions of classes)Family (subdivisions of orders)

Genus (subdivisions of families)Species (distinct type of life)

Figure 9-14 ■ The shape and structure of antenna are used to separate butterflies from moths.(a) The antennae of butterflies end in knobs; (b) those of moths either have no knobs or arefeather shaped.

9-5 What Is Happening to Earth’s Biodiversity? 265

Figure 9-15 ■ Going, Going, Gone. (a) Whooping cranes are making a slow recovery in NorthAmerica. (b) The last authenticated sighting of an ivory-billed woodpecker in the United Stateswas in 1971. (c) The last two great auks were shot in 1844.

2. Today, biologists recognize three Domains:

◆ Archaea (ancient bacteria with prokaryotic cells)◆ Bacteria (true bacteria, also with prokaryotic cells)◆ Eukarya (organisms with eukaryotic cells)

3. The old Kingdom Monera (single-celled, prokaryotic organisms) has recently beensplit into the other two Domains, Archaea and Bacteria. The number and descrip-tions of kingdoms within these domains are currently in a state of flux.

4. The Domain Eukarya currently has four kingdoms: Protista (single-celled and sim-ple multicelled eukaryotes); Fungi (single-celled and multicelled, eukaryotic, het-erotrophic organisms with thick-walled cells); Plantae (multicelluar, eukaryotic,autotrophic organisms with thick-walled cells); and Animalia (multicellular, eu-karyotic, heterotrophic organisms with cells that have no walls). Subdivisions of eachof the kingdoms are discussed in the next section.

9-5 What Is Happening to Earth’s Biodiversity?

Something is happening to the diversity of life on Earth. Faster than most of us realize,it is disappearing. So far in this chapter, we have focused on how science handles the myr-iad of species that make up Earth’s biodiversity—the system of classification with whichscientists organize, keep track of, and determine relationships between species. In Ap-pendix C, we review major highlights of this vast, cumbersome, complex, elegant, and sur-prisingly workable system. In this section, we shift focus to a more practical topic. It’s atopic many biologists see as a developing crisis. It’s a problem whose solution—and thegood news is that there is a solution—will involve, not only scientists, but also politi-cians, theologians, and everyday citizens.

The problem is that life on Earth is disappearing (Figure 9-15).Today, there is onlya fraction of the estimated 75 million bison that greeted the first Europeans who cameto the Great Plains of North America. Today, the numbers of songbirds are consistent-ly down throughout the world. Today, many once extensively forested areas are com-pletely devoid of trees.Today, many species are simply disappearing. Indeed, in the timeit has taken you to read this section thus far, it is quite likely that another of Earth’s

(a) (b) (c)


species—another distinct form of life; another of evolution’s success stories—has be-come extinct. It may have been a bird, a mammal, or some other vertebrate. It was morelikely an insect, simply because there are more of them. Or most likely of all, this now-extinct species was an unseen, unknown, unstudied invertebrate or plant that lived andthrived for perhaps a million years or more, and now it is gone.

Forever.Extinctions are nothing new. Indeed, most species become extinct naturally. But the

rate at which extinctions are now occurring suggests that we are entering a mass extinctionevent.These, too, are not new. Scientists have identified five previous mass extinctions inEarth’s history.The latest and best-studied occurred 65 million years ago, when dinosaursand many other species became extinct. So, if extinction is natural and if life has alwayssurvived even mass extinction events, why are we concerned? First, although new specieswill eventually evolve to replace those lost during even a mass extinction, the recoveryperiod is measured in tens of millions of years. Do we wish to impose that long a periodof impoverished biodiversity on our descendants? Second, in previous mass extinctions,the forms of life dominant at the time of the event became extinct.Ammonites, trilobites,and dinosaurs are all gone. The fact that we are now dominant may give us reason topause.The question is not will life survive—of course it will.The question is will we, as aspecies, survive, and what will be the quality of life for our descendants. Finally, whereasprevious mass extinctions were caused by nonbiological factors (meteorites probablycaused the dinosaurs’ extinction), this current event is being caused by a biologicalimbalance. Out of the 100 million or so species one has become not only dominant, butis exercising its ability to modify its own environment and destroy other species on agrand and massive scale. That species is Homo sapiens.

Human Activities Are Causing Mass ExtinctionsWhat are we doing to cause this state of affairs? The root cause of the problem—indeedthe root cause of all environmental problems—is human population growth. In Chapter16, we will discuss more fully this fascinating and important topic. Here, we will con-centrate on how our population impinges on other organisms.

We are the most cosmopolitan of species; that is, we live nearly everywhere.Whereveranything lives, especially on land, we are able to fit in.And as our numbers grow, we findourselves in competition with other organisms for space and other resources. Since we arestronger (at least as a group), smarter, and better organized, we invariably win; the othersinvariably lose. What we do impinges on other species. Specific examples follow.

Traditionally, overexploitation has been a major factor in reducing Earth’s biodi-versity. We are dependent on other species for much of what we need, especially forfood.As our numbers increase, the pressures we put on these species increases apace andoften outstrips the exploited species’ ability to reproduce. Today, this is seen most crit-ically in oceanic fishery stocks. Modern boats and other technologies allow us to exploitfish stocks anywhere in the world. Wherever large populations of fish are found, boatsand fishermen are sent out to harvest them.As more and more fishermen are attractedto the profitable site, pressure on the resource mounts. Soon, the population crashes.Atfirst, reluctant to give up their lucrative life’s work, fishermen continue to harvest, andpressures on the resource increase. Eventually, usually within a few years, the fish become“economically extinct”—that is, unprofitable to harvest, and the industry moves on, torepeat the pattern on another fishery. Sometimes the now-decimated population re-covers. More often than not, by the time the fishermen move on, numbers have been soreduced that recovery is slow and sometimes impossible. Today, we eat fish our grand-parents never heard of, such as orange roughy and tilapia, because the fish they ate, codand haddock for example, are all but gone (Figure 9-16).

Other species are exploited for reasons other than food. Exotic birds and aquariumfish are sometimes harvested in the wild at unsustainable rates. Elephant populationswere decimated for their ivory. Similarly, exotic plants, such as orchids, were overhar-


vested to become houseplants far from their lands of origin. In tropical countries, teakand other tree species have been harvested to the point of extinction.

Habitat loss is another problem for many species. As our numbers grow, so does thesize of our towns and cities. Fifty years ago, cities of over 1 million were rare.Today, Mex-ico City, Mexico, and Shanghai, China, vie for the title of world’s largest city—at over 20million people each. Shanghai occupies a territory roughly the size of the state of Delaware.A few wild species are able to adjust to the urban environment; the vast majority are not.

Compounding the problem is our growing need for farmland.As our numbers grow,so does our need for food, most of which comes from farms. Especially near cities, whatonce was habitat for wildlife and plants is rapidly converted into farmland. Further-more, the amount of land needed to provide food for a city is vastly larger than the cityitself.Within agricultural lands, few wild animals and plants are tolerated. Most are seenas pests, threats, or competitors and are eliminated.

Various infrastructures take other tolls on wildlife habitat. Roadways, railways, pow-erline rights-of-way, and the like connect cities and consume habitat. Dams built to con-trol floods, assist in irrigation, and provide hydroelectric power inundate additional landand displace additional wildlife.

And what happens to animals and plants displaced by development? Perhaps a fewcan move into surrounding territories. But these territories quickly fill, if they were notfull to begin with. Most of the displaced simply die.

Figure 9-16 ■ Fish for the Table—Then and Now: (a) Haddock, (b) Cod, (c) Tilapia,(d) Orange Roughy.

(a) (b)

(c) (d)


Chemicals produced by human activities and released into the natural environmentmay be adversely affecting the reproductive rates of animals and thus adversely affect-ing their populations. As a society, we are dependent on many such chemicals. Plastics,fuels, solvents, cleaning compounds, drugs—the list could go on and on. Once used, manyof these chemicals leak or are purposely put into soils, waterways, or the atmosphere,where many degrade slowly if at all. Here they accumulate, sometimes to toxic levels.Here, animals and plants incidentally consume these chemicals and are exposed to theiraffects. How serious is the problem? We’re not sure, and finding answers is an immenseproblem.An advisory group for the Environmental Protection Agency is currently try-ing to determine the possible environmental effects of 87,000 separate chemicals.Whereto begin? How to proceed? Their task is monumental.

Meanwhile, examples of animals being affected by these chemicals are being doc-umented regularly. Chemicals that mimic hormones are blamed for cancers, ulcers, andother deformities in freshwater fish and beluga whales. Misshapen reproductive organswere found in Florida alligators believed to have consumed a pesticide accidentallyspilled into their lake. Other reproductive abnormalities observed in gulls, mink, eagles,and other animals have been blamed on exposure to chemicals in their environments.Other examples, particularly in more remote parts of the world, probably go undetect-ed. Humans are not immune. Exposure to chemicals resembling estrogen, a female re-productive hormone, may be adversely affecting sperm counts in populations of humanmales throughout the world.

Alien species—those that flourish in regions where they are not native—are a grow-ing, complex, and immense problem (Figure 9-17). Species travel. Some such travel is per-fectly natural, an integral part of evolution. (Review Chapter 2.) But in today’s world,most alien species stow away in ships, trucks, cars, planes, and trains. Others, such as petsand ornamental plants, escape from homes, gardens, and yards. Most aliens find envi-ronmental conditions in the new territories intolerable and die out. Still others—indeed,a surprising number—thrive in the new territory, often replacing native species andoverwhelming local resources. North America alone is thought to host over 5,000 alienspecies, and more are arriving every year.

Some of our alien species are welcome and necessary. Honeybees pollinate most ofthe crop plants consumed by Americans. Other alien species are not so welcome, but donot cause serious problems. Dandelions are a scourge of the modern yard, but are notan environmental crisis. Some alien species are, however. Purple loosestrife, still sold insome nurseries, escapes the garden and overwhelms local wetlands, to the detriment ofnative wetland plants and the waterfowl that feed upon them. Snakehead fish, import-ed from China primarily for the pet trade, have been released in the wild and are caus-ing serious problems to native fish populations in several states. Zebra mussels, nativeto the Balkans, were apparently accidentally released into the Great Lakes from theballast tanks of cargo ships in the 1980s.They are having huge economic impacts and arereplacing native mollusks throughout the Great Lakes. More worrisome, they are spread-ing south, east, and west. Kudzu, a vine native to Japan, was introduced into the UnitedStates in 1876 as an ornamental. Its growth rate is phenomenal—up to a foot a day dur-ing summer months and up to 60 feet a year. Growing conditions in the southeasternUnited States are ideal, where, out of the garden, it overgrows and eventually destroyswhole forests of plants and animals.

The horror stories could go on and on. Preserving native species and habitats bycontrolling alien species is becoming a serious challenge throughout the world.

These are a few of the human activities that adversely affect native plants and ani-mals. Often these “stressors” act in consort. Species whose habitats are being degradedby encroaching development and whose reproductive rates are being reduced by expo-sure to chemical pollutants are not capable of successfully competing with recently in-troduced aliens.To help these species, solutions must be sought that will relieve all of thechallenges they face simultaneously.


Humans Are Dependent on Healthy Populations of Other OrganismsSpecies need each other. Indeed, no species can live alone, and we are no exception. (SeeChapters 15 and 16.) While extensive use of technology, synthetic fabrics, and domesti-cated crop species have somewhat lessened our dependence on the natural world, weare still dependent on other organisms to provide services we cannot provide for ourselves.

In at least one area, our dependence is still absolute—the air we breathe. Here weare involved in an elegant partnership. Plants convert the carbon dioxide we produce intothe oxygen we cannot live without.And we produce the carbon dioxide that they use inphotosynthesis. (See Chapter 10.) In addition to what we exhale with every breath, we

Figure 9-17 ■ Unwanted Aliens in North America: (a) Kudzu is native to Japan; (b) Zebramussels are native to the Balkans, (c) Snakehead fish are native to China.

(a)

(b) (c)


produce additional carbon dioxide with the fuels we burn.As we shall learn in Chapter16, the buildup of carbon dioxide and other gases from human activities is causing Earth’satmosphere to warm to alarming levels.To remove carbon dioxide from the atmosphere,we turn to those organisms that can do photosynthesis—green plants and algae. Onland, extensive forests of large trees are the most effective, least expensive means ofaccomplishing this vital function. Our dependence on trees is absolute and growing daily.

Today, many of our cities and farmlands are flood prone. In the United States, almostany serious rain is accompanied with flash flood warnings that all too often materialize intofloods. Throughout the Caribbean and in Central and South America, serious mudslidesare increasingly common. Traditionally, floods and mudslides in these areas were rare.What has changed? Often the culprit is deforestation. In cities and farmlands, where treesare rare, most rain that falls quickly runs off into rivers and streams, often overwhelmingtheir banks. Runoff is even more serious on steep, denuded slopes. In both cases, plants,particularly trees with deep root systems, slow runoff, soak up and use at least some of thewater, and prevent tragedies. Forests that provide us with flood and mudslide control alsoprovide habitat for whole communities of wildlife—at no additional expense to humans.

As our numbers grow, so do the wastes we produce. Many accidentally leak or arepurposely put into our rivers and streams. From there they flow into our lakes andoceans. Polluted water not only affects wildlife, but can become a health hazard to hu-mans. For many municipalities, particularly in Europe and North America, the solutionis technological. Large, expensive water and sewage treatment plants turn foul watercoming out of cities into acceptably clean water released back to nature. Wetlands canoften accomplish the same task for a fraction of the cost. Many municipalities are cre-ating new wetlands, reclaiming old ones, and diverting others from development for theexpress purpose of treating polluted water.The cost savings is significant. In rural areas,when given a chance, wetlands can remove excess pesticide and fertilizer runoff fromfarms. Thus preserved, wetlands, too, provide quality habitat for wildlife.

Who would have thought, nearly 100 years ago, that a lowly fungus, best known forspoiling fruit and bread stored a little too long in damp, dark places, would hold a secretwonder drug? In 1929,Alexander Fleming discovered that a common household fungalpest, called Penicillium, produced a product that could kill several varieties of disease-causing bacteria. The discovery was important for two reasons. First, it gave rise to awhole new paradigm for treating diseases, the use of antibiotics. Second, it illustrated thateven some of Earth’s lowliest creatures can produce products useful to humans. Sincethen, hundreds of antibiotics and other drugs have been discovered, isolated, and utilizedfrom natural sources. Organisms other than fungi are often important. For example,in 1962, it was discovered that an extract from the bark of the yew, a lowly “weed tree”common in the Pacific Northwest, can control numerous human cancers. What otherorganisms harbor cures for cancer or similar, useful products? We, of course, don’t know.The fact that there could be many has stimulated a whole new career in biology.Today, bioprospectors are combing through remote and not so remote areas, seekinghitherto unsuspected, potentially useful products associated with what most of us mightconsider unlikely candidate organisms. One thing is clear: If an organism becomes ext-inct before its potential usefulness can be determined, it’s too late. Here is one of the mostcompelling arguments for preserving Earth’s biodiversity.

Finally, there is the question of ethics. Each species alive today is the result of a longand successful evolutionary history.We, of course, consider ourselves to be the most im-portant species on Earth. But from an ecological standpoint, no one species is moreimportant than any other.True, some play minor roles and some play major roles, but allare important.What right, then, do we have to rob other species of continued existence?We are the only species who can understand the implications of our actions. Along withthat awesome capability comes the equally awesome responsibility to control our activitiesfor the benefit of all life. Our survival may well depend on how well we do.Thoreau said,“In wilderness is the preservation of the world”—our world as well as the natural world.And let us always remember, healthy wilderness requires extensive biodiversity.


We Can Preserve Our BiodiversityAs dire as the biodiversity crisis seems the good news is that solutions are possible andat hand. If current trends continue, species will be lost. But current trends need notcontinue. Human activities can be directed to building up rather than tearing down ourenvironment. Sooner or later, it is hoped, doing so will become a high enough priorityin public and private policy that real progress can be made. Preserving biodiversity willbe part of that process.

Traditionally, we have focused efforts on preserving individual species. Game andsimilar laws were passed to restrict harvests of game fish, mammals, and birds—often withlimited success.For example, in the 1930s,populations of bald eagle—our nation’s symbol—were falling rapidly. In 1940, a law was passed prohibiting all killing of bald eagles. Still, theirnumbers fell. It wasn’t until laws were passed that preserved and improved the quality oftheir habitat that eagle numbers started to once again increase. Here is a real success story.Today, throughout much of their former range, bald eagles have returned. The lessonlearned:The best way to preserve species of interest is to preserve their habitat.

Success Stories The peregrine falcon in North America is another species that has madea successful recovery in recent years. How was this done? What are some other similar suc-cess stories? Try doing a web search to find out.

Preserving habitat means establishing and maintaining parks and refuges designedspecifically for wildlife. Throughout North America, an extensive system of parks andrefuges exists. The situation is less sanguine throughout much of the rest of the world.Wherever parks and refuges exist, pressures to convert them into cities or farmland areenormous.Their continued existence as parks and refuges requires constant vigilance bythose interested in habitat and species preservation.

Often, parks and refuges become islands of natural biodiversity surrounded by asea of development. Generally speaking, the larger the island, the more effective it is inproviding habitat. But even small “islands” can be useful to wildlife, especially if joinedto similar areas with narrow corridors allowing species passage between areas. Oftenthe process is twofold: Establish a system of parks, then link them with corridors.

Restoration ecology is becoming an important career path for those interested in theenvironment.The goal is to transform spent mines, worn-out farmland, deforested slopes,even unprofitable shopping centers, and other lands tainted with human activities intowildlife habitat. Wetlands are being restored. Forests and natural grasslands are beingreplanted. New parks and refuges are being created on lands that today harbor no orscant wildlife. Thus, native plants and animals are being reintroduced into lands wherethey have been missing. Restoration ecology is an exciting and challenging new way toeffectively preserve biodiversity.

Perhaps the most important factor in preserving biodiversity is citizen involvement.There is a role in this issue for everyone to play. Homeowners can replace today’s typ-ical yard of monotonous grass-plus-one-tree-or-bush with an expanse of native plants thatwill provide habitat for native butterflies, songbirds, and other wildlife. Such yards re-quire no fertilizers, no pesticides, and little watering. Everyday citizens can join supportgroups that do volunteer and advocacy work on existing parks and refuges and lobby forthe establishment of new ones. Vacationers can plan to visit parks and refuges, both athome and abroad. Ecotourism, where local people make their living showing travelersthe wonders of their native wildlife, is becoming one of the most important factors in pre-serving biodiversity worldwide. Voters can choose candidates at least partially on thebasis of their interest in and knowledge of environmental issues. Everyone can get in-volved. Remember the words of anthropologist Margaret Meade, “Never doubt that asmall group of thoughtful, committed citizens can change the world. Indeed, it is theonly thing that ever has.” Biodiversity needs you.


What Can I Do to Help? Describe some of the many ways that private citizens can becomeinvolved in preserving biodiversity. Try searching the web for “citizen science,” “BackyardHabitat,” and “Audubon at Home.”

WHERE ARE WE NOW?Taxonomy and systematics are currently two of the most active and exciting areas incontemporary biology. There are at least three reasons for this:

1. New technologies are being adopted by systematists that allow more precise deter-minations of relationships between species. Borrowing techniques widely used inbiochemistry, they are now able to determine (a) the order of amino acids in proteins,(b) the order of nucleotides in ribosomal RNA and mitochondrial DNA, and (c) par-tial and complete genomes of specific organisms. These are the most basic levels atwhich existing species become new ones. Species with similar protein and nucleicacid structures are assumed to be more closely related than those with dissimilari-ties. Furthermore, since the rate is known at which mutations change these factors,the length of time two species have been evolutionarily separated can be estimated.Old assumptions are being challenged, some verified, others revised.

2. The growing awareness that biological classification is only just beginning has stim-ulated increased interest in the field. Estimates now place the total number of Earth’sspecies at 100 million, of which only about 1.5 million have been described. Biolo-gists are finding new species every day, and opportunities to do so will continuethroughout most of this century.

3. At the same time, there is a sense of urgency within the field. Human activities, par-ticularly in the tropics, are negatively affecting biodiversity (Figure 9-18). Indeed,some authorities believe that Earth is entering a new “mass extinction event” thatwill see the loss of millions of species. At present, species are becoming extinctevery day, some before scientists can describe them. Unless they leave fossils, thesespecies will never be added to the list of Earth’s species.Their importance to ecosys-tems will never be fully appreciated. Perhaps most important of all, as they pass

Figure 9-18 ■ Human activities arehaving a negative impact on Earth’sbiodiversity. As more and more forests arecut to be turned into farms, shopping malls,housing developments, and so on, moreand more species are becoming extinct.

Review Questions 273

REVIEW QUESTIONS

1. Where might life on Earth have originated? (Hint: There are sev-eral possible answers to this question.)

2. Describe the major milestones in the evolution of life on Earth.3. Approximately how many species have been described by biol-

ogists? How is that number likely to change in the future?4. How are taxonomy and systematics similar? How are they different?5. The classification system used by biologists is said to be hierar-

chical and comparative. What does this mean?6. Arrange the following taxa in order, starting with the most

inclusive (largest):class family kingdom phylumdomain genus order species

7. Why is the scientific name of humans, Homo sapiens, written theway it is?

8. What are the purposes of biological classification?9. Today, biologists recognize three domains.What are they? What

are the distinguishing characteristics of each?10. For a long time there were only two kingdoms, plants and ani-

mals. Why was it necessary to create more kingdoms?11. What are the major taxa associated with humans?

12. What are the principle causes of extinction in today’s world?13. Why are alien species generally problematic?14. How can Earth’s biodiversity be preserved?

Answers to questions 15–21 can be found in Appendix C.

15. What are the characteristics shared by all bacteria? Briefly dis-cuss the range of variations that exist among bacteria.

16. What characteristics are shared by all protists? What are the majorgroups of protists? What distinguishes one group from another?

17. What are the characteristics shared by all fungi?18. What are lichens? Why are they difficult to classify?19. What are some of the characteristics shared by all plants? What

are the major divisions of plants? What distinguishes each divi-sion from the others?

20. What characteristics are shared by all animals? Briefly discussthe major animal phyla. How is each phylum different from theothers?

21. Think of each of the kingdoms and the major phyla within each.Which of these groups of organisms made the transition fromwater to land?

into extinction, their potential contribution to human welfare is forever lost. Somemay contain cures for human diseases, be potential new foods for humans, or pro-duce otherwise useful products.Taxonomists are being urged to go into areas whereextinctions are thought to be occurring most rapidly in sometimes desperate effortsto catalog species and save remnant populations before they are lost.

Stemming the Tide What is being done to reverse the tide of extinction that seems to beaffecting species everywhere? What more needs to be done? How much will it cost? Are thebenefits worth the cost?

Exploration

9 biodiversitymyresource.phoenix.edu/secure/resource/bio100r3...240 chapter 9 biodiversity: how...

Documents