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Chapter 7
Evolution—The Theory and Its
Supporting Evidence
Darwin and the Galápagos
• During Charles Darwin’s five-year voyage
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–
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–
–
(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
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
The Galápagos Islands
• The Galápagos
Islands
– are specks of
land
– composed of
basalt
– in the eastern
Pacific
Darwin Developed the Theory
• During the voyage Darwin observed
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–
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–
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–
–
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
Galápagos Finches
• Darwin’s finches from the Galápagos Islands
– arranged to show evolutionary relationships
Insect eaters
Insect eaters
Berry
eater
Seed Cactus
eaters eaters
– Notice
that beak
shape
– varies
depending
on diet
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
– encyclopedia collection of facts
• Evolution provides a framework
– for discussion of life history
Evolution: Historical Background
• 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 and the works of Aristotle
– 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 the Bible
• 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
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
• Lamarck’s theory seemed logical at the time
Lamarck’s Theory
• 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
during its lifetime
Darwin
• In 1859, Charles
Robert Darwin (18091882)
– published On the
Origin of Species
• in which he detailed
– his ideas on evolution
– formulated 20 years
earlier
– and proposed a
mechanism for
evolution
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 Malthus’ essay on population
– suggested that competition for resources
– and high infant mortality limited population size
Artificial Selection
• Through artificial selection
• humans have giving rise to dozens of varieties of
– domestic dogs, pigeons, sheep, cereal crops, and vegetables
• Wild mustard was selectively bred
– to yield broccoli, cauliflower, kale, and cabbage
Darwin and Wallace
• 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
Natural Selection—Main Points
• 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
• Those with favorable variations
– are more likely to survive
– and pass on their favorable variations
“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 inheritable variations
– leading to reproductive success
Alternate Uses for Structures
• Critics claim that complex features
–
–
–
–
such as eyes and bird’s wings
could not have evolved by natural selection
Anything less than the existing structure
would be useless
• The “to complex to have evolved” argument
– fails because it assumes that
– structures have always been used
– exactly as they are now
Not only Biggest,
Strongest, Fastest
• One misconception about natural selection
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–
–
–
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
–
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–
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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...
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
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
Mendel’s 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)
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
Importance of Mendel’s 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
Genes and Chromosomes
• Complex, double-stranded
helical molecules
– of deoxyribonucleic acid (DNA)
• called chromosomes
– are found in cells of organisms
• 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
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
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
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
Mitosis
• Mitosis is cell division
– that results in
– the complete duplication of a
cell
• In this example,
– a cell with four
chromosomes (two pairs)
– produces two cells
– each with four chromosomes
• Mitosis takes place
– in all cells except sex cells
Mitosis
• Once an egg
– has been fertilized,
– the developing embryo
– grows by mitosis
Modern View of Evolution
• During the 1930s and 1940s,
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–
–
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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
• They completely rejected Lamarck’s idea
– of inheritance of acquired characteristics
• They reaffirmed the importance of natural
selection
• According to modern evolutionary theory,
– populations rather than individuals evolve.
– Individuals with favorable traits
• are more likely to survive and reproduce
• if their variations are favorable
– As a result descendant populations possess variations in
greater frequency
What Brings about Variation?
• Evolution by natural selection
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works on variation in populations
most of which is accounted for by the reshuffling
of genes from generation to generation
during sexual reproduction
• The potential for variation is enormous
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–
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–
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
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
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
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
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
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
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
Allopatric 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!
Speciation in Songbirds
• Two species of Eurasian songbirds
– appear to have evolved from an ancestral species
• Adjacent populations (A & B, F & G) can
interbreed
– But E and H cannot interbreed
Rate of Speciation
• Although widespread agreement exists
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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
– 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 as little as a few thousands of years
Styles of Evolution
• 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
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
Convergent Evolution
• Convergent evolution takes place
– when distantly related organisms give rise to
species
– that resemble
one another
– because they
adapt
– in comparable
ways
Parallel Evolution
• Parallel evolution
– involves the independent
origin
– of similar features in related
organisms
Microevolution and Macroevolution
• 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
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
Phylogenetic Tree
• A phylogenetic tree
– showing the
relationships
– among various
vertebrate animals
Cladogram
• A cladogram showing inferred relationships
• Some of the characteristics used
– to construct this cladogram are indicated
Evolutionary Novelties
• All land-dwelling vertebrate animals
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possess bone and paired limbs
so these characteristics are primitive
and of little use in establishing relationships
among land vertebrates
• However, hair and three middle ear bones
– are derived characteristics
– because only one subclade, the mammals, has them
Evolutionary Novelties
• If considering only mammals,
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hair and middle ear bones
are primitive characteristics,
but live birth is a derived characteristic
that serves to distinguish most mammals
from the egg-laying mammals
Cladograms
• 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
• Dogs and birds
– appear closely related
• Hair and giving birth to live
young
– indicate that bats and dogs
– are more closely related
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
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
Evolutionary Trends
• Evolutionary changes do not involve
– all aspects of an organism 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
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
Evolutionary Trends
• Abundant fossils show the evolutionary trends of
– the Eocene mammals, 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 shown
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
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
– whose shell has not changed
– significantly since the Ordovician
“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
– Latrimaria (fish)
– Gingko trees
• Some of these are generalized and can live
under a wide variety of environments
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
A Second Living Fossil
• Ginkgos
– have changed very
little
– for millions of
years
Randomness in Natural Selection?
• 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?
Two Steps in Natural Selection
• Evolution by natural selection
– is a two-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
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.
– 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
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 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
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
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
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
Theories Must Be Predictive
• Not all predictions are about future events, and
– evolutionary theory cannot make predictions
– about the far distant future
• Nevertheless, we can make a number of
predictions
–
–
–
–
about the present-day biological world
and about the fossil record
that should be consistent with evolutionary theory
if it is correct
Some Predictions from Evolution
• If evolution has taken place,
– closely related species such as wolves and coyotes
– should be similar in anatomy and biochemistry,
– genetics, and embryonic development
Testable
• 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
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
• Linnaeus’s classification is an ordered list
– of categories that becomes more inclusive
– as one proceeds up the list
Linnaean Classification
• the coyote, Canis latrans
• Animalia
– Chordata
Most inclusive
• Kingdom
– Phylum
• Subphylum
– Class
» Order
• Family
– Genus
• Species
Least inclusive
• Vertebrata
– Mammalia
» Carnivora
• Canidae
– Canis
• latrans
Classification —shared Characteristics
• Subphylum
vertebrata
– including
fishes,
amphibians,
reptiles, birds
and mammals,
– have a
segmented
vertebral
column
• Only warmblooded
animals with
hair/fur and
mammary
glands are
mammals
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
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
Biological Evidence
Supporting Evolution
• 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
Evolutionary Relationships
• 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 tests support the idea
– that birds descended from reptiles
• a conclusion supported by evidence in the fossil record
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
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 similarly
arranged with respect to other structures,
– have similar pattern of embryonic development
Analogous Structures
• Wings of insects and birds
– serve the same function but differ considerably
– in structure and development
Vestigial Structures
• Vestigial structures are 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
Remnants of Rear Limbs in Whales
• The Eocene-aged whale,
Basilosaurus,
– had tiny vestigial back
limbs
– but it did not use limbs to
support its body weight.
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
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
Advent of Various Vertebrates
• Times
when
major
groups of
vertebrates
appeared
in the
fossil
record
• Thickness
of spindles
shows
relative
abundance
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
Horses and Their Relatives
• 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
Missing Links
• Missing links
– or fossils bridging gaps
– between presumed ancestor and descendant groups
of organisms
• are claimed by some non-scientists to invalidate
the theory of evolution
• There are many intermediate fossils
– The transition from fish to amphibians is well
documented by fossils
– Some of the most advanced mammal-like reptiles
and earliest mammals are very similar