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