Evolution—The Theory and Its Supporting Evidence

Chapter 7

-ideas change thru time: Lamarck to Darwin to Mendel…..-voyage of HMS Beagle: Galapagos Islands…finches….

allopatric speciation- isolation of populations-phyletic gradualism vs punctuated equilibrium

-Evolution- an explanation that ties disparate events together… ‘survival of the fittest’….bit of a misnomer…

-’Living fossils’ today: coelocanth and ginkgo-Mass extinctions ….in geologic past-Structure and function of limbs: homologous, analogous, and vestigial-The Linnean biological classification: ….species…genus…

• Some of the evidence for evolution – is provided by fossils – such as this Early

Pleistocene mammoth– known as Archidiskodon

meridonalis • on display in the Museum

of Geology and Paleontology at the University of Florence in Italy

Evidence for Evolution

• The fossil record consists – of first appearances of various organisms – through time

• One-celled organisms appeared – before multi-celled ones– plants appeared before animals– invertebrates before vertebrates

• Fish appeared first followed – in succession by amphibians, – reptiles, mammals, and birds

What do We Learn from Fossils?

• Times when major groups of vertebrates appeared in the fossil record

• Thickness of spindles shows relative abundance

Advent of Various Vertebrates

• Fossils are much more common – than many people realize

• However the origin and initial diversification – of a group is generally the most poorly represented

• But fossils showing the diversification – of horses, rhinoceroses, and tapirs – from a common ancestor are known

• as are ones showing the origin – of birds from reptiles

• and the evolution – of whales from a land-dwelling ancestor

Fossils Are Common

• During Charles Darwin’s five-year voyage – (1831-1836) on the HMS Beagle, – he visited the Galápagos Islands – where he made important observations – that changed his ideas about – the then popular concept called the fixity of species

• an idea holding that all present-day species • had been created in their present form • and had changed little or not at all

• Darwin fully accepted – the Biblical account of creation before the voyage

Darwin and the Galápagos

• Map showing the route (red line) followed – by Charles Darwin when he was aboard – HMS Beagle from 1831 to 1836

• The Galápagos Islands – are in the Pacific Ocean west of Ecuador

Route of HMS Beagle

The Galápagos Islands

• The Galápagos Islands – are specks of land – composed of basalt – in the eastern Pacific

• During the voyage Darwin observed – that fossil mammals in South America – are similar, yet different from present-day – llamas, sloths, and armadillos– that the finches and giant tortoises living – on the Galápagos Islands vary from island to island – and still resemble ones from South America, – even though they differ in subtle ways

• These observations convinced Darwin – that organisms descended with modification – from ancestors that lived during the past– the central claim of the theory of evolution

Darwin Developed the Theory

• Evolution – involving inheritable changes in organisms through

time • is fundamental to biology and paleontology

– Paleontology is the study of life history as revealed by fossils

• Evolution is a unifying theory • like plate tectonic theory

– that explains an otherwise – encyclopedic collection of facts

• Evolution provides a framework – for discussion of life history – in later parts of the term

Why Study Evolution?

• Many people have a poor understanding – of the theory of evolution – and hold a number of misconceptions, – which include:

• evolution proceeds strictly by chance

• nothing less than fully developed structures – such as eyes are of any use

– there are no transitional fossils -so-called missing links

• connecting ancestors and descendants

• humans evolved from monkeys – so monkeys should no longer exist

Misconceptions about Evolution

• Evolution, the idea that today’s organisms – have descended with modification – from ancestors that lived during the past,

• is usually attributed solely to Charles Darwin, – but it was seriously considered long before he was

born,– even by some ancient Greeks– and by philosophers and theologians

• during the Middle Ages

• Nevertheless, the prevailing belief – in the 1700s was that Genesis – explained the origin of life – and contrary views were heresy

Evolution: Historical Background

• During the 18th century, – naturalists were discovering evidence – that could not be reconciled – with literal reading of Scripture

• In this changing intellectual atmosphere, – scientists gradually accepted a number of ideas:

• the principle of uniformitarianism,• Earth’s great age,• that many types of plants and animals had become extinct,• and that change from one species to another occurred

• What was lacking, though, – was a theoretical framework to explain evolution

Evolution: Historical Background

• Jean-Baptiste de Lamarck – (1744-1829) is best remembered for his theory – of inheritance of acquired characteristics, – even though he greatly contributed – to our understanding of the natural world

• According to this theory, – new traits arise in organisms because of their needs – and are somehow passed on to their descendants

• Lamarck’s theory seemed logical at the time – and was widely accepted

Lamarck

• Lamark’s theory was not totally refuted – until decades later – with the discovery that genes

• units of heredity

– cannot be altered by any effort by an organism

Lamarck’s Theory

• According to Lamarck’s theory of inheritance of acquired characteristics

Lamarck’s Giraffes

– ancestral short-necked giraffes

– stretched their necks

– to reach leaves high on trees

– their offspring were born

– with longer necks

• In 1859, Charles Robert Darwin (1809-

1882) – published On the

Origin of Species

• In it he detailed – his ideas on evolution – formulated 20 years

earlier– and proposed a

mechanism for evolution

Darwin

• Plant and animal breeders – practice artificial selection – by selecting those traits they deem desirable – and then breed plants and animals with those traits– thereby bringing about a great amount of change

• Observing artificial selection – gave Darwin the idea that – a process of selection among variant types – in nature could also bring about change

• Thomas Malthus’s essay on population – suggested that competition for resources – and high infant mortality limited population size

Natural Selection

• Darwin and Alfred Russell Wallace (1823-1913) – read Malthus’s book – and came to the same conclusion,

• that a natural process – was selecting only a few individuals for survival

• Darwin’s and Wallace’s idea– called natural selection– was presented simultaneously in 1859

Darwin and Wallace

• Organisms in all populations – possess heritable variations such as– size, speed, agility, visual acuity, – digestive enzymes, color, and so forth

• Some variations are more favorable than others– some have a competitive edge – in acquiring resources and/or avoiding predators

• Not all young survive to reproductive maturity– Those with favorable variations – are more likely to survive – and pass on their favorable variations

Natural Selection—Main Points

• According to the Darwin-Wallace theory – of natural selection, giraffe’s long neck evolved

Naturally Selected Giraffes

– because ancestors with longer necks

– had an advantage

– and reproduced more often

• In colloquial usage, – natural selection is sometimes expressed as – “survival of the fittest”

• This is misleading because– natural selection is not simply a matter of survival– but involves differential rates – of survival and reproduction

“Survival of the Fittest”

• One misconception about natural selection – is that among animals– only the biggest, strongest, and fastest – are likely to survive– These characteristics might provide an advantage

• but natural selection may favor – the smallest if resources are limited– the most easily concealed– those that adapt most readily to a new food source– those having the ability to detoxify some substance– and so on...

Not only Biggest, Strongest, Fastest

• Natural selection works – on existing variation in a population

• It could not account for the origin of variations• Critics reasoned that should a variant trait arise,

– it would blend with other traits and would be lost

• The answer to these criticisms – existed even then in the work of Gregor Mendel, – but remained obscure until 1900

Limits of Natural Selection

• If a species is well adapted to its environment, – most mutations would not be particularly useful – and perhaps would be harmful

• But what was a harmful mutation – can become a useful one – if the environment changes

• Information in cells is carried on chromosomes – which direct the formation of proteins – by selecting the appropriate amino acids – and arranging them into a specific sequence

• Neutral mutations may occur – if the information carried on the chromosome – does not change the amino acid or protein – that is produced

Mutations

• Some mutations are induced by mutagens– agents that bring about higher mutations rates such

as• some chemicals• ultraviolet radiation• X-rays• extreme temperature changes

• Some mutations are spontaneous– occurring without any known mutagen

What Causes Mutations?

• Species is a biological term for a population – of similar individuals that in nature interbreed – and produce fertile offspring

• Species are reproductively isolated – from one another

• Goats and sheep do not interbreed in nature, – so they are separate species

• Yet in captivity – they can produce fertile offspring

Species

• Speciation is the phenomenon of a new species – arising from an ancestral species

• It involves change in the genetic makeup – of a population, – which also may bring about changes – in form and structure

• During allopatric speciation, – species arise when a small part of a population – becomes isolated from its parent population

Speciation

• Reduction of the area occupied by a species – may leave a small isolated population– Two peripheral isolates evolved into new species– Isolation might result from a marine transgression.

Allopatric Speciation

• Geographic barriers may form across parts – of a central population’s range, – thereby isolating small populations that speciate

Allopatric Speciation

Finch Speciation • Darwin’s finches from the Galápagos Islands – underwent allopatric

speciation – due to isolation of

birds among the many islands

Insect eaters

Berry eater

Seed eaters

Cactus eaters

• During the 1860s, Gregor Mendel, • an Austrian monk,

– performed a series of controlled experiments – with true-breeding strains of garden peas– strains that when self-fertilized – always display the same trait, such as flower color

• Traits are controlled by a pair of factors, – now called genes

• Genes occur in alternate forms, called alleles– One allele may be dominant over another– Offspring receive one allele – of each pair from each parent

Mendel and the Birth of Genetics

• The parental generation consisted of – true-breeding strains, RR = red flowers, rr = white flowers

• Cross-fertilization yielded a second generation – all with the Rr combination of alleles,

• in which the R (red) is dominant over r (white)

Mendel’s Experiments

• The second generation, when self-fertilized – produced a third generation – with a ratio of three red-flowered plants – to one white-flowered plant

Mendel’s Experiments

• The factors (genes) controlling traits – do not blend during inheritance

• Traits not expressed in each generation – may not be lost

• Therefore, some variation in populations – results from alternate expressions of genes (alleles)

• Variation can be maintained

Importance of Mendel’s Work

• called chromosomes– are found in cells of all organisms

• except bacteria, • which have ribonucleic acid (RNA)

• Specific segments of DNA – are the basic units of heredity

(genes)• The number of chromosomes

– varies from one species to another– fruit flies 8; humans 46; horses 64

Genes and Chromosomes• Complex, double-stranded

helical molecules – of deoxyribonucleic acid (DNA)

• In sexually reproducing organisms, – the production of sex cells

• pollen and ovules in plants • sperm and eggs in animals

– results when cells undergo a type of cell division – known as meiosis

• This process yields cells – with only one chromosome of each pair– so all sex cells have – only 1/2 the chromosome number – of the parent cell

Sexually Reproducing Organisms

• During meiosis, – sex cells form that

contain one member – of each chromosome

pair

• Formation of sperm is shown here

• Eggs form the same way, – but only one of the

four final eggs – is functional

Meiosis

• The full number of chromosomes – is restored when a sperm fertilizes an egg

Fertilization

– or when pollen fertilizes an ovule

• The egg (or ovule) then – has a full set of

chromosomes – typical for that species

• As Mendel deduced, – 1/2 the genetic makeup – of fertilized egg – comes from each parent

• The fertilized egg – grows by mitosis

• Mitosis is cell division – that results in – the complete duplication of a

cell• In this example,

– a cell with four chromosomes (two pairs)

– produce two cells– each with four chromosomes

• Mitosis takes place – in all cells except sex cells

Mitosis

• Evolution by natural selection – works on variation in populations– most of which is accounted for by the reshuffling – of alleles from generation to generation – during sexual reproduction

• The potential for variation is enormous– with thousands of genes – each with several alleles, – and with offspring receiving 1/2 of their genes – from each parent

• New variations arise by mutations– change in the chromosomes or genes

What Brings about Variation?

• Mutations result in a change – in hereditary information

• Mutations that take place in sex cells – are inheritable,– whether they are chromosomal mutations

• affecting a large segment of a chromosome

– or point mutations• individual changes in particular genes

• Mutations are random with respect to fitness– they may be beneficial, neutral, or harmful

Mutations

• Although widespread agreement exists – on allopatric speciation– scientists disagree on how rapidly – a new species might evolve– Phyletic gradualism

Rate of Speciation

• the gradual accumulation of minor changes

• This view was held by Darwin and reaffirmed by modern synthesis

• eventually brings about the origin of new species

• Punctuated equilibrium

Rate of Speciation

– holds that little or no change

– takes place in a species – during most of its

existence

– giving rise to a new species

– in perhaps as little as a few thousand years

– then evolution occurs rapidly

• Divergent evolution occurs – when an ancestral species– giving rise to diverse descendants – adapts to various aspects of the environment

• Divergent evolution leads to descendants – that differ markedly from their ancestors

• Convergent evolution involves the development – of similar characteristics – in distantly related organisms

• Parallel evolution involves the development – of similar characteristics – in closely related organisms

Styles of Evolution

• Divergent evolution of a variety – of placental mammals from a common ancestor

• Divergence accounts for descendants – that differ from their ancestors and from one another

Divergent Evolution

• Convergent evolution takes place – when distantly related organisms give rise to

species

Convergent Evolution

– that resemble one another

– because they adapt

– in comparable ways

• Parallel evolution – involves the independent

origin – of similar feature in related

organisms

Parallel Evolution

• A cladogram showing inferred relationships• Some of the characteristics used

– to construct this cladogram are indicated

Cladogram

• Phylogeny is the evolutionary history – of a group of organisms

• If sufficient fossil material is available, – paleontologists determine the phylogeny – and evolutionary trends for groups of organisms

• For example, one trend in ammonoids• extinct relatives of squid and octopus

– was the evolution – of an increasingly complex shell

Phylogeny

• Abundant fossils show the evolutionary trends of– the Eocene mammals family Brontotheridea,

• better known as titanotheres

Evolutionary Trends

• These extinct relative of horses and rhinoceroses – evolved from small ancestors – to giants standing 2.4 m at

the shoulder– developed large horns– and the shape of their skull

changed– Only 4 of the 16 known

genera are show

• Size increase is – one of the most common evolutionary trends

• However, trends are complex– they might reverse– more than one can take place – at the same time at different rates

• Trends in horses included – generally larger size

• but size decreased in some now-extinct horses– changes in teeth and skull – lengthening legs – reduction in number of toes

• These trends occurred at different rates

Evolutionary Trends

• Several organisms have shown – little or no change for long periods

• If these still exist as living organisms today – they are sometimes called living fossils

• For example:– horseshoe crabs

• closest living relative of a trilobite

– coelacanth (fish)– gingkoes (tree)

• The absence of change for these organisms – is not yet fully understood

“Living Fossils”

• Latimeria – belongs to a group of fish – once thought to have gone extinct – at the end of the Mesozoic Era

A specimen was caught off the coast of East Africa in

1938

A Living Fossil

A Second Living Fossil• Ginkgos

– have changed very little

– for millions of years

• They were found– living in some

isolated habitats in Asia

– and have been transplanted elsewhere

• Perhaps as many as 99% of all species – that ever existed are now extinct

• Organisms do not always evolve – toward some kind of higher order of perfection – or greater complexity

• Vertebrates are more complex – but not necessarily superior – in some survival sense than bacteria– after all, bacteria have persisted – for at least 3.5 billion years

• Natural selection yields organisms adapted – to a specific set of circumstances – at a particular time

Extinctions

• The continual extinction of species – is referred to as background extinction

• It is clearly different from mass extinction– during which accelerated extinction rates – sharply reduce Earth’s biotic diversity

• Extinction is a continual occurrence– but so is the evolution of new species – that usually quickly exploit the opportunities – another species’ extinction creates

• Mammals began a remarkable diversification – when they began occupying niches – the extinction of dinosaurs and their relatives left

vacant

Background and Mass Extinction

• The mass extinction of dinosaurs – and other animals at the end of Mesozoic Era – is well known,

• but the greatest mass extinction – occurred at the end of the Paleozoic Era

• More than 90% of all species died out– We will discuss these extinctions – and their possible causes later in the term

Mass Extinction

• Darwin cited supporting evidence – for evolutionary theory such as

• classification• embryology• comparative anatomy• geographic distribution• fossil record, to a limited extent

• He had little knowledge– of the mechanism of inheritance – and biochemistry and molecular biology – were unknown at his time

Evidence in Support of Evolution

• Since Darwin’s time, studies from additional fields – in biochemistry– molecular biology– more complete and better understood fossil record

• have convinced scientists that the theory – is as well supported by evidence – as any other major theory

• Scientists still disagree on many details, – but the central claim of the theory – is well established and widely accepted

Evidence in Support of Evolution

• An idea can only be a truly scientific theory – if testable predictive statements – can be made from it

• No theory in science is ever proven • in the final sense,

– although substantial evidence may support it

• All theories are always open – to question, revision and occasionally – to replacement by a more comprehensive theory

Is the Theory of Evolution Scientific?

• By predictive, we do not mean that – it can predict the future

• No one knows which existing species – will become extinct, or what descendants – of any particular organism, if any, – will look like in 10 million years from now

• Nevertheless, we can make a number of predictions – about the present-day biological world– and about the fossil record – that should be consistent with the theory of

evolution, – if it is correct

Theories Must Be Predictive

• If evolution has taken place, – the oldest fossil-bearing rocks should have

– very different fossils than organisms of today

• More recent rocks should have – more fossils similar to today’s organisms

• Closely related species should have similarities – in a whole range of areas, not just anatomy

Some Predictions from Evolution

• Classification of organisms – should show a nested pattern of similarities

• Neighboring plants and animals – should be more similar to each other – than to ones farther away

• A mechanism should exist – that allows the evolution of one species to another– fossils should appear in the fossil record – in order of the organisms’ evolution

Some Predictions from Evolution

• Suppose that contrary to evolutionary prediction – wolves and coyotes were not similar – in terms of their biochemistry, genetics – and embryonic development– Our prediction would fail – and we would at least have to modify the theory

• If other predictions also failed– say, if mammals appeared in the fossil record before

fishes– then we would have to abandon the theory – and find a better explanation for our observations

• Since the theory of evolution is testable, – it is truly scientific

Testable

• If all existing organisms actually evolved – from ancestors that lived during the past,

• all life forms should have fundamental similarities:– all living things consist mainly of carbon, nitrogen

hydrogen and oxygen– their chromosomes consist of DNA

• except bacteria which have RNA

– all cells synthesize proteins – in essentially the same way

Biological Evidence Supporting Evolution

• Biochemistry provides evidence – for evolutionary relationships

• Blood chemistry is similar among all mammals – Humans’ blood chemistry is related

• most closely to the great apes• then to Old World monkeys• then New World monkeys• then lower primates such as lemurs

• Biochemical test support the idea – that birds descended from reptiles

• a relationship also evidenced in the fossil record

Evolutionary Relationships

• Homologous structures – are basically similar structures – that have been modified for different functions– They indicate derivation from a common ancestor.

• Analogous structures are structures – with similarities unrelated – to evolutionary relationships – that serve the same function– but are quite dissimilar – in both structure and development

Structures with Similarities

• Forelimbs of humans, whales, dogs, and birds – are superficially dissimilar,– yet all are made up of the same bones,

Homologous Structures

– have similar arrangement

– of muscles, nerves and blood vessels,

– are similarlyarranged with respect to other structures,

– have similar pattern of embryonic development

• Wings of insects, birds and bats – serve the same function but differ considerably – in structure and embryological development

• Are any of these wings – both analogous and homologous?

Analogous Structures

• Yes, bird and bat wings

• Vestigial structures are nonfunctional remnants – of structures in organisms that were functional – in their ancestors

Vestigial Structures

• Why do dogs have tiny, – functionless toes on their

feet (dewclaws)?• Ancestral dogs had five

toes – on each foot, – all of which contacted the

ground

• As they evolved – they became toe-walkers with only four toes on the ground – and the big toes and thumbs were lost or reduced – to their present state

• Classification uses a nested pattern of similarities

• Carolus Linneaus (1707-1778) proposed – a classification scheme – in which organisms receive a two-part name – consisting of genus and species– for example, the coyote is Canis latrans

• Linnaeus’s classification is an ordered list – of categories that becomes more inclusive – as one proceeds up the hierarchy

Classification

• Kingdom– Phylum

• Subphylum– Class

» Order

Linnaean Classification

• the coyote, Canis latrans• Animalia

– Chordata• Vertebrata

– Mammalia» Carnivora

• Canidae– Canis

• latrans

• Family– Genus

• Species

Most inclusive

Least inclusive

• Subphylum vertebrata – including

fishes, amphibians, reptiles, birds and mammals,

– have a segmented vertebral column

• Only warm-blooded animals with hair/fur and mammary glands are mammals

Classification —shared Characteristics

• Coyote (Canis latrans) and wolf (Canis lupus) – share numerous characteristics – as members of the same genus

• They share some but fewer characteristics – with the red fox (Volpes fulva) – in the family Canidae

• All canids share some characteristics with cats, – bears and weasels in the order Carnivora– which is one of 18 living orders – of the class Mammalia

• Shared characteristics – are evidence for evolutionary relationships

Coyote and Wolf

• Small-scale evolution can be observed today.• For example

– adaptations of some plants to contaminated soils– insects and rodents developing resistance to new

insecticides and pesticides– development of antibiotic-resistant strains of

bacteria

• Variations in these populations – allowed some variant types – to live and reproduce, – bringing about a genetic change

Evolution in Living Organisms

• This cladogram shows the relationship among – tapirs, rhinoceroses, horses and their extinct relative – the titanotheres and chalicothers– which are well documented by fossils

Horses and Their Relatives

• These might seem an odd assortment of animals – but fossils and studies of living animals – indicate that they shared a common ancestor

• As we trace these animals back – in the fossil record, – differentiating one from the other – becomes increasingly difficult

• The earliest members of each group – are remarkably similar, – differing mostly in size and details of their teeth

• As their diversification proceeded – the differences became more apparent

Horses and Their Relatives

• Of course, we will never have enough fossils – to document the evolutionary history – of all living creatures simply because fossilization – is an incomplete process

• The remains of some organisms – are more likely to be preserved than those of others– and accumulation of sediments – varies in both space and time

• But several other kinds of evidence – support the concept of evolution – including biochemistry, comparative anatomy, – genetics, molecular biology, – and small-scale evolution of living organisms

Never Enough

Summary

• Jean Baptiste de Lamarck proposed – the first formal theory of evolution – to be taken seriously– Inheritance of acquired characteristics – was his mechanism for evolution

• In 1859 Charles Robert Darwin – and Alfred Russel Wallace – published their views on evolution, – and proposed natural selection – as the mechanism for evolutionary change

Summary

• Gregor Mendel’s breeding experiments – with garden peas provided some of the answers – regarding how variation – is maintained and passed on– Mendel’s work is the basis for modern genetics

• Genes are the hereditary determinants – in all organisms

• This genetic information is carried – in the chromosomes of cells– Only the genes in – the chromosomes of sex cells are inheritable

Summary

• Sexual reproduction and mutations – account for most variation in populations

• Evolution by natural selection has 2-steps – First, variation must be produced – and maintained in interbreeding populations, – and second, favorable variants – must be selected for survival

• An important way by which new species evolve – is allopathic speciation

Summary

• When a group is isolated – from its parent population, – gene flow is restricted or eliminated, – and the isolated group is subjected – to different selection pressures

• Divergent evolution involves – an ancestral stock giving rise – to diverse species

• The development of similar adaptive types – in different groups of organisms results – from parallel and convergent evolution

Summary

• Scientists are increasingly using – cladistic analyses to determine relationships – among organism, – but they still show relationships– using phylogenetic trees

• Extinctions take place continually, – and times of mass extinctions – resulting in marked decreases – in Earth’s biologic diversity – have occurred several times

Summary

• The theory of evolution is truly scientific – because we can make observations

– that would falsify it

– That is, it could conceivably be proved wrong

• Much of the evidence supporting – the theory of evolution comes from

– classification, embryology, genetics,

– biochemistry, molecular biology,

– and present-day small-scale evolution

Summary

• The fossil record also provides evidence – for evolution in that it shows a sequence – of different groups appearing through time, – and some fossils show features – we would expect in the ancestors of birds – or mammals, and so on

The Galápagos Islands

• Information in cells is carried on chromosomes – which direct the formation of proteins – by selecting the appropriate amino acids – and arranging them into a specific sequence

• Neutral mutations may occur – if the information carried on the chromosome – does not change the amino acid or protein – that is produced

Neutral Mutations

• During evolution, all aspects of an organism – do not change simultaneously

• A key feature we associate – with a descendant group might appear – before other features typical of that group

• For example, the oldest known bird – had feathers and the typical fused clavicles of birds, – but it also retained many reptile characteristics

• Mosaic evolution is the concept that – organisms possess recently evolved characteristics – as well as some features of their ancestral group

Evolutionary Trends

• Normally a horse’s back foot – has only one functional toe, – the third

Remnants of Toes in Horses

• Splints are small – remnants of toes 2 and 4 – that remain as vestiges

• Occasionally, – horses are born – with one or both – of these vestiges enlarged

• Traditionally, scientists have – depicted evolutionary relationships – with phylogenetic trees

• in which the horizontal axis represents • anatomical differences • and the vertical axis denotes time

• In contrast, a cladogram shows – the relationships among members of a clade

• a group of organisms • including its most recent common ancestor

• Cladistics focus on derived characteristics • sometimes called evolutionary novelties

– as opposed to primitive characteristics

Cladistics and Cladograms

• A phylogenetic tree – showing the

relationships – among various

vertebrate animals

Phylogenetic Tree

• All land-dwelling vertebrate animals – posses bone and paired limbs – so these characteristics are primitive – and of little use in establishing relationships – among land vertebrates

• However, hair and mammary glands – are derived characteristics– Only one subclade, the mammals, has them

Evolutionary Novelties

• If considering only mammals, – hair and mammary glands – are primitive characteristics, – but live birth is a derived characteristic – that serves to distinguish most mammals – from the egg-laying mammals

Evolutionary Novelties

• Three different interpretations – of the relationships among – bats, dogs and birds

Cladograms

• Bats and birds fly, – which might suggest – a closer relationship – than to dogs

Cladograms

• Dogs and birds – do not appear closely related

• Hair and giving birth to live young – indicate that bats and dogs – are more closely related

• Darwin’s finches from the Galápagos Islands – arranged to show evolutionary relationships

Galápagos Finches

– Notice that beak shape

– varies depending on diet

Berry

eater

Insect eaters

Insect eaters

Cactus eaters

Seed eaters

• In allopatric speciation – a small population may evolve – whereas the larger parent population may

• remain unchanged,

• evolve in some other direction,

• or become extinct

Various Possibilities

• In both convergent and parallel evolution,– similar characteristics developed independently – in comparable environments

Styles of Evolution

• Once an egg – has been fertilized, – the developing embryo – grows by mitosis

Mitosis

• During the 1930s and 1940s, – paleontologists, population biologists, – geneticists, and others developed ideas that– merged to form a modern synthesis – or neo-Darwinian view of evolution

• They incorporated – chromosome theory of inheritance – into evolutionary thinking

• They saw changes in genes (mutations) – as one source of variation

Modern View of Evolution

• A few individuals may somehow reach – a remote area and no longer exchange genes – with the parent population– This out-migration can lead to the formation

• of a peripheral isolate that gives rise to a new species

– while the parent population persists without change

Allopatric Speciation

• Ideas about speciation – commonly involve misconceptions

• One anti-evolution argument is – “If humans evolved from monkeys, – “why are there still monkeys?”

• This involves two misconceptions– No scientist has ever claimed

• that humans evolved from monkeys

– Even if they had, that would not preclude • the possibility of monkeys still existing

Misconceptions

• They completely rejected Lamarck’s idea – of inheritance of acquired characteristics

• They reaffirmed the importance of natural selection

• But since then, – some scientists have challenged the emphasis

– in modern synthesis

– that evolution is gradual

Modern View of Evolution

• Cladistics and cladograms work – well for living organisms, – but are trickier for fossils

• Care must be taken in determining – what are primitive verses derived characteristics, – especially in groups with poor fossil records

• Paleontologists must be especially careful – of characteristics resulting – from convergent evolution

Cladistics for Fossils

• Nevertheless, cladistics is a powerful tool – that has more clearly elucidated – the relationships among many fossil lineages, – and is now used extensively by paleontologists

Cladistics for Fossils

• 18 orders of mammals exist including order Carnivora

• The Family Canidae are doglike carnivores

• and the genus Canis includes only closely related species

• Coyote, Canis latrans, stands alone as a species

Coyote, Canis latrans

• But isn’t evolution by natural selection – a random process?

• If so, how is it possible – for a trend to continue long enough – to account just by chance – for such complex structures as – eyes, wings, and hands?

Randomness in Natural Selection?

• Evolution by natural selection – is a 2 step process– Only the first step involves chance

• Variation must be present – or arise in a population

• Whether a mutation is favorable – is a matter of chance

• The natural selection of favorable variations – is not by chance

Two Steps in Natural Selection

• Evolutionary trends are a series of adaptations– to changing environment– or in response to exploitation of new

habitats• Some organisms

– show little evolutionary change – for long periods

• Lingula is a brachiopod – with a shell, at least, – that has not changed – significantly since the Ordovician

Adaptations