Evolution

1
Chapter 7
EvolutionThe Theory and Its Supporting Evidence
2
Evidence for Evolution

• 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

3
Darwin and the Galápagos

• During Charles Darwins 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

4
Route of HMS Beagle

• 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

5
The Galápagos Islands

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

6
The Galápagos Islands
7
Darwin Developed the Theory

• 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

8
Galápagos Finches

• Darwins finches from the Galápagos Islands
• arranged to show evolutionary relationships

• Notice that beak shape
• varies depending on diet

Berry eater
Seed eaters
Cactus eaters
9
Why Study Evolution?

• 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

10
Misconceptions about 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

11
Evolution Historical Background

• Evolution, the idea that todays 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

12
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,
• Earths 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

13
Lamarck

• 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
• Lamarcks theory seemed logical at the time
• and was widely accepted

14
Lamarcks Theory

• Lamarks 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

15
Lamarcks Giraffes

• According to Lamarcks theory of inheritance of
  acquired characteristics

• ancestral short-necked giraffes
• stretched their necks
• to reach leaves high on trees
• their offspring were born
• with longer necks

16
Darwin

• 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

17
Natural Selection

• 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 Malthuss essay on population
• suggested that competition for resources
• and high infant mortality limited population size

18
Darwin and Wallace

• Darwin and Alfred Russel Wallace (1823-1913)
• read Malthuss book
• and came to the same conclusion,
• that a natural process
• was selecting only a few individuals for survival
• Darwins and Wallaces idea
• called natural selection
• was presented simultaneously in 1859

19
Natural SelectionMain Points

• Organisms in all populations
• posses 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

20
Naturally Selected Giraffes

• According to the Darwin-Wallace theory
• of natural selection, giraffes long neck evolved

• because ancestors with longer necks
• had an advantage
• and reproduced more often

21
Survival of the Fittest

• 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

22
Not only Biggest, Strongest, Fastest

• 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...

23
Limits of Natural Selection

• 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

24
Mendel and the Birth of Genetics

• 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

25
Mendels Experiments

• 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)

26
Mendels Experiments

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

27
Importance of Mendels Work

• 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

28
Genes and Chromosomes

• Complex, double-stranded helical molecules
• of deoxyribonucleic acid (DNA)

• 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

29
Sexually Reproducing Organisms

• 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

30
Meiosis

• 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

31
Fertilization

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

• 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

32
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

33
Mitosis

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

34
Modern View of Evolution

• 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

35
Modern View of Evolution

• They completely rejected Lamarcks 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

36
What Brings about Variation?

• 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

37
Mutations

• 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

38
Mutations

• 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

39
Neutral Mutations

• 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

40
What Causes 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

41
Species

• 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

42
Speciation

• 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

43
Allopatric 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.

44
Allopatric Speciation

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

45
Allopatric Speciation

• 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

46
Finch Speciation

• Darwins finches from the Galápagos Islands
• underwent allopatric speciation
• due to isolation of birds among the many islands

47
Rate of Speciation

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

• the gradual accumulation of minor changes

• eventually brings about the origin of new species

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

48
Rate of Speciation

• Punctuated equilibrium

• holds that little or no change
• takes place in a species
• during most of its existence

• then evolution occurs rapidly

• giving rise to a new species
• in perhaps as little as a few thousand years

49
Misconceptions

• Ideas about speciation
• commonly involve misconceptions
• One antievolution 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

50
Various Possibilities

• In allopatric speciation
• a small population may evolve
• whereas the larger parent population may
• remain unchanged,
• evolve in some other direction,
• or become extinct

51
Styles of Evolution

• 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

52
Styles of Evolution

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

53
Divergent 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

54
Convergent Evolution

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

• that resemble one another
• because they adapt
• in comparable ways

55
Parallel Evolution

• Parallel evolution
• involves the independent origin
• of similar feature in related organisms

56
Cladistics and Cladograms

• 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

57
Phylogenetic Tree

• A phylogenetic tree
• showing the relationships
• among various vertebrate animals

58
Cladogram

• A cladogram showing inferred relationships
• Some of the characteristics used
• to construct this cladogram are indicated

59
Evolutionary Novelties

• 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

60
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

61
Cladograms

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

62
Cladograms

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

• Dogs and birds
• do not appear closely related

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

63
Cladistics for Fossils

• 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

64
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

65
Evolutionary Trends

• 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

66
Phylogeny

• 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

67
Evolutionary Trends

• Abundant fossils show the evolutionary trends of
• the Eocene mammals family Brontotheridea,
• better known as titanotheres

• 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

68
Evolutionary Trends

• 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

69
Adaptations

• 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

70
Living Fossils

• 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

71
A Living Fossil

• 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

72
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

73
Randomness in Natural Selection?

• But isnt 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?

74
Two Steps 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

75
Extinctions

• 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

76
Background and Mass Extinction

• 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 Earths 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

77
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

78
Evidence in Support of Evolution

• 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

79
Evidence in Support of Evolution

• Since Darwins 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

80
Is the Theory of Evolution Scientific?

• 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

81
Theories Must Be Predictive

• 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

82
Some Predictions from Evolution

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

83
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

84
Testable

• 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

85
Classification

• 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
• Linnaeuss classification is an ordered list
• of categories that becomes more inclusive
• as one proceeds up the hierarchy

86
Linnaean Classification

• the coyote, Canis latrans
• Animalia
• Chordata
• Vertebrata
• Mammalia
• Carnivora

• Most inclusive

• Kingdom
• Phylum
• Subphylum
• Class
• Order

• Canidae
• Canis
• latrans

• Family
• Genus
• Species

Least inclusive
87
Classification shared Characteristics

• 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

88
Coyote, Canis latrans

• 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

89
Coyote and Wolf

• 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

90
Biological Evidence Supporting Evolution

• 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

91
Evolutionary Relationships

• 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

92
Structures with Similarities

• 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

93
Homologous Structures

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

• have similar arrangement
• of muscles, nerves and blood vessels,

• are similarlyarranged with respect to other
  structures,
• have similar pattern of embryonic development

94
Analogous Structures

• 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?

• Yes, bird and bat wings

95
Vestigial Structures

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

• 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

96
Remnants of Toes in Horses

• Normally a horses back foot
• has only one functional toe,
• the third

• 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

97
Evolution in Living Organisms

• 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

98
What do We Learn from Fossils?

• 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

99
Advent of Various Vertebrates

• Times when major groups of vertebrates appeared
  in the fossil record
• Thickness of spindles shows relative abundance

100
Fossils Are Common

• 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

101
Horses and Their Relatives

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

102
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

103
Never Enough

• 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

104
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

105
Summary

• Gregor Mendels breeding experiments
• with garden peas provided some of the answers
• regarding how variation
• is maintained and passed on
• Mendels 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

106
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

107
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

108
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 Earths biologic diversity
• have occurred several times

109
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

110
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