Evolution—The Theory and Its Supporting Evidence

Chapter 7

• Some of the evidence for evolution – comes from fossils

– such as this 2.3-meter-long Ceresiosaurus

– which belonged to a groupof Triassic marine reptiles known as nothosaurs.

• On display at Glacier Garden, Lucerne, Switzerland

Evidence for Evolution

• 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 – are specks of

land – composed of

basalt – in the eastern

Pacific

The Galápagos Islands

The Galápagos Islands

• 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

• 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

• 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

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

• In 1859, Charles Robert Darwin (1809-

1882) – published On the

Origin of Species

• in which 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’ essay on population – suggested that competition for resources – and high infant mortality limited population size

Natural Selection

• Darwin and Alfred Russel Wallace (1823-1913) – read Malthus’ 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 over produce• Inheritable variations are passed down 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

• Successful Reproduction of their favorable variations

Natural Selection—4 Steps

• 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

• 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

• 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

• A few individuals of a species on the mainland– reach isolated island 1– Speciation follows genetic divergence in a new

habitat.

Allopatric Speciation

• Later in time, a few individuals of the new species colonize island 2– In this new habitat, speciation follows genetic

divergence.

Allopatric Speciation

• Speciation may also follow colonization of islands 3 and 4

• Invasion of island 1 by genetically different descendants of the ancestral species!

Allopatric Speciation

• More than 20 species of Hawaiian honeycreepers have evolved– from a common ancestor as they adapted to diverse food

sources on the islands

Honeycreeper Speciation

• Divergent evolution occurs – when an ancestral species– gives rise to diverse descendants – adapted 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

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

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 features in related

organisms

Parallel Evolution

• Microevolution is any change in the – the genetic make-up of a species, and – involves changes within a species

• Macroevolution involves changes– such as the origin of a new species– or changes at even higher levels – For example, the origin of birds from reptiles

• The cumulative effects of microevolution – are responsible for macroevolution

Microevolution and Macroevolution

• 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

• 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 – Latrimaria (fish)– Gingko trees

• Some of these are generalized and can live under a wide variety of enviroinments

“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

• 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, 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– when more than 90% of all species died out

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 millions of years from now

• Nevertheless, we can make a number of predictions – about the present-day natural world– and about the fossil record – that should be consistent with evolutionary theory – if it is correct

Theories Must Be Predictive

• If evolution has taken place, – closely related species such as wolves and coyotes

– should be similar in anatomy and biochemistry,

– genetics, and embryonic development

• The oldest fossil-bearing rocks should have – very different fossils than organisms of today

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, then– our prediction would fail – and we would at least have to modify the theory

• If other predictions also failed,– for example, 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 can “be falsified,” – it is truly scientific

Testable

• 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

• 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

• 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

• If all existing organisms descended with modification – 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– all cells synthesize proteins

• in essentially the same way

Biological Evidence Supporting Evolution

• Biochemistry provides evidence – for evolutionary relationships

• Blood proteins are 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 conclusion supported by evidence 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

• The Eocene-aged whale, Basilosaurus, – had tiny vestigial back

limbs – but it did not use limbs to

support its body weight.

Remnants of Rear Limbs in Whales

• 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

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

• One-celled organisms appeared – before multicelled 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

• This cladogram shows the relationship among – tapirs, rhinoceroses, and horses

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