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