1. Evolution—The Theory and Its Supporting Evidence
Chapter 7
Evolution—The Theory and Its Supporting Evidence

2. Darwin and the Galápagos
During Charles Darwin’s five-year voyage
( ) 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

3. 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

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

5. 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

6. Galápagos Finches Darwin’s finches from the Galápagos Islands
arranged to show evolutionary relationships
Notice that beak shape
varies depending on diet
Berry eater
Insect eaters
Seed eaters
Cactus eaters

7. 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

8. 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

9. 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

10. Lamarck Jean-Baptiste de Lamarck According to this theory,
( ) 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

11. 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

12. Darwin In 1859, Charles Robert Darwin (1809-1882) in which he detailed
published On the Origin of Species
in which he detailed
his ideas on evolution
formulated 20 years earlier
and proposed a mechanism for evolution

13. 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

14. 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

15. 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

16. 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

17. “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

18. 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

19. 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...

20. 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

21. 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

22. 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)

23. 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

24. 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

25. 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

26. 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

27. Meiosis During meiosis, Formation of sperm is shown here
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

28. Fertilization The full number of chromosomes The egg (or ovule) then
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

29. Mitosis Mitosis is cell division In this example, Mitosis takes place
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

30. Mitosis Once an egg has been fertilized, the developing embryo
grows by mitosis

31. 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

32. 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

33. What Brings about Variation?
Evolution by natural selection
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
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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. Allopatric Speciation
A few individuals of a species on the mainland
reach isolated island 1
Speciation follows genetic divergence in a new habitat.

41. Allopatric Speciation
Later in time, a few individuals of the new species colonize island 2
In this new habitat, speciation follows genetic divergence.

42. Allopatric Speciation
Speciation may also follow colonization of islands 3 and 4
Invasion of island 1 by genetically different descendants of the ancestral species!

43. 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

44. 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
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

45. 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
in closely related organisms

46. Styles of Evolution In both convergent and parallel evolution,
similar characteristics developed independently
in comparable environments

47. 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

48. Convergent Evolution Convergent evolution takes place
when distantly related organisms give rise to species
that resemble one another
because they adapt
in comparable ways

49. Parallel Evolution Parallel evolution involves the independent origin
of similar features in related organisms

50. 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

51. 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

52. Phylogenetic Tree A phylogenetic tree showing the relationships
among various vertebrate animals

53. Cladogram A cladogram showing inferred relationships
Some of the characteristics used
to construct this cladogram are indicated

54. Evolutionary Novelties
All land-dwelling vertebrate animals
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

55. Evolutionary Novelties
If considering only mammals,
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

56. Cladograms Three different interpretations of the relationships among
bats, dogs and birds

57. Cladograms Bats and birds fly, Dogs and birds
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

58. 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

59. 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

60. 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

61. 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

62. 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

63. Evolutionary Trends Size increase is However, trends are complex
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

64. 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

65. “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

66. 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

67. A Second Living Fossil Ginkgos have changed very little
for millions of years

68. 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?

69. 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

70. 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

71. 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

72. 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

73. 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

74. 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

75. 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

76. 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

77. 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

78. Classification Classification uses a nested pattern of similarities
Carolus Linneaus ( ) 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

79. Linnaean Classification
Most inclusive
the coyote, Canis latrans
Animalia
Chordata
Vertebrata
Mammalia
Carnivora
Kingdom
Phylum
Subphylum
Class
Order
Canidae
Canis
latrans
Family
Genus
Species
Least inclusive

80. 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

81. 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

82. 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

83. 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

84. 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

85. 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

86. 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

87. Analogous Structures Wings of insects and birds
serve the same function but differ considerably
in structure and development

88. 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

89. 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.

90. 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

91. 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

92. Advent of Various Vertebrates
Times when major groups of vertebrates appeared in the fossil record
Thickness of spindles shows relative abundance

93. 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

94. Horses and Their Relatives
This cladogram shows the relationship among
tapirs, rhinoceroses, and horses

95. 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

96. 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

97. The Evidence: Summary Scientists agree that the theory of evolution is
as well-supported by evidence
as any other theory
Transitional fossils provide compelling evidence
but fossils aren’t the only evidence
Much evidence comes from
comparative anatomy
biogeography
molecule biology
genetics
embryology
and biochemistry

98. Summary The central claim of evolution is that all organisms
have descended with modification
from ancestors that lived during the past.
Jean Baptiste de Lamarck proposed
the first mechanism to
account for evolution
Inheritance of Acquired Characteristics
Darwin’s observation of variation in populations and artificial selection
and his reading of Malthus’ essay on population
helped him formulate his idea of natural selection

99. Summary In 1859 Charles Darwin and Alfred Wallace
published their ideas of natural selection
which hold that in populations of organisms,
some have favorable traits that make it
more likely that they will survive and reproduce
natural selection

100. Summary Gregor Mendel’s breeding experiments
with garden peas provided some of the answers
regarding how variation
is maintained in populations
Mendel’s work is the basis for present-day genetics
Genes are the hereditary units
in all organisms
Only the genes in sex cells are inheritable

101. Summary Sexual reproduction and mutations
account for most variation in populations
Evolution by natural selection has two steps
First, variation must exist or arise
and be maintained in interbreeding populations,
and second, favorable variants
must be selected for survival

102. Summary Most species evolve by allopatric speciation
which involves isolation of a small population from its parent population,
that is then subjected to different selection pressures
Divergent evolution involves
A common ancestor stock giving rise
to diverse species
The development of similar adaptive types
in different groups of organisms results
from parallel and convergent evolution

103. Summary Microevolution involves changes within a species,
while microevolution encompasses all changes above the species level.
Scientists traditionally used phylogenetic trees
to depict evolutionary relationships
but now they more commonly use
cladistic analyses and cladograms
to show these relationships

104. Summary Background extinctions occur continually,
but several mass extinctions have also taken place
during which Earth’s biologic diversity
has decreased markedly
The theory of evolution is truly scientific
because we can make observations
that would falsify it
That is, prove it wrong

105. Summary Much of the evidence supporting
the theory of evolution comes from
classification, comparative anatomy,
embryology, genetics, biochemistry,
molecular biology, and present-day examples of microevolution
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, horses, whales, and so on