Download 122 [Study Guide] 22-2 Evidence for Evolution

Study Guide
Evidence for Evolution
Checking Your Knowledge
☛ You should be able to write out the definitions to each of the following terms in your own words:
predator
drug resistance
fossil record
radiometric dating
homologous structure
vestigial structure
evolutionary tree
convergent evolution
biogeography
continental drift
endemic species
Evolution is supported by an overwhelming amount of scientific evidence.
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Darwin’s The Origin of Species presented a broad range of evidence to
support the idea of descent with modification.
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Four types of data provide evidence for the existence of evolution and tell us
about how it occurs:
1. direct observations of evolution;
2. the fossil record;
3. homology; and
4. biogeography.
Direct observations of evolutionary change
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Evolution serves as the core theme in biology because it is able to explain diverse data from many
fields of biology.
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Let’s consider two examples of natural selection as a mechanism of evolution in populations.
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The first example concerns predation of guppy populations.
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Predation (being eaten by other organisms) is a potent force shaping the adaptations of prey.
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Predators are most likely to feed on prey that do not hide, run away, or defend themselves.
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Prey that are eaten are less likely to reproduce and pass their traits on to their offspring than
those individuals who evade predation.
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You probably know guppies (Poecilia reticulata) as popular aquarium fish, but they also live wild in
pools in the Aripo River system in Trinidad.
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John Endler observed that the color patterns of male guppies are extremely variable: No two males
look alike.
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Female guppies are attracted to brightly colored males and are more likely to choose them as mates
than to choose males with drab coloring.
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Unfortunately, brightly colored males are more attractive to predators.
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If a guppy population contained both brightly colored and drab males, predators might be
expected to eat more of the brightly colored ones.
Endler hypothesized that males faced a trade-off between attracting mates and attracting predators.
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In the field, Endler observed that the color patterns of male guppies corresponded to the
intensity of predation.
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In pools with few predators, male guppies tended to be very bright.
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In pools with many predator species, males were drab.
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Based on these observations, Endler hypothesized that more intense predation caused natural
selection to favor the trait of drab coloration in male guppies.
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He tested this hypothesis by adding brightly colored guppies to a pool with many predators.
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As he predicted, over time the transplanted guppy population became less brightly colored.
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Endler also transferred guppies with drab colors (from a pool with many predators) to a pool with
few predators.
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In their new environment, brightly colored males rapidly came to dominate the guppy population.
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Endler’s experiments demonstrate that natural selection can cause rapid evolution in wild
populations.
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A second example of ongoing natural selection is the evolution of drug-resistant pathogens (diseasecausing organisms).
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The evolution of drug resistance is a particular problem in
bacteria and viruses, which reproduce rapidly.
•
HIV (human immunodeficiency virus) is the virus that causes
AIDS.
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Researchers have developed numerous drugs to treat HIV,
but using these medications selects for viruses resistant to
the drugs.
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There is a chance that a few drug-resistant viruses are
present at the beginning of treatment.
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The drug-resistant pathogens are more likely to survive
treatment and pass on the genes that enable them to
resist the drug to their offspring.
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As a result, the frequency of drug resistance in the viral population rapidly increases.
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Checking Your Understanding
☛ Based on the graph, were the three patients in better shape at Week 8 than they were at the
beginning of treatment? Why or why not?
☛ Explain why it is incorrect to state that anti-HIV drugs have created drug resistance in the virus.
•
The examples of the guppies and drug-resistant viruses highlight two important points about natural
selection:
1. Natural selection is an editing mechanism, not a creative force. It can act only on the existing
variation in the population; it cannot create favorable traits, it selects for favorable traits that are
already present in the population.
2. Natural selection favors traits that increase fitness in the current, local environment. What is
adaptive in one situation is not adaptive in another.
◼
For example, male guppies with drab colors are at an advantage in a pool with many
predators, but at a disadvantage in a pool with few predators.
The fossil record
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The fossil record shows that various groups of organisms have changed dramatically in form over
time.
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Fossils also document the origin of major groups of
organisms.
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For example, an increasingly good fossil record shows
the early evolution of cetaceans, the group of mammals
that includes whales, dolphins, and porpoises.
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The earliest cetaceans lived 50 to 60 million years ago.
Before that time, most mammals were terrestrial (lived
on land).
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Over the last several decades, a series of fossils have
been found that demonstrate the transition from
terrestrial to aquatic life, leading to the loss of hind limbs
and the development of flippers.
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The fossil record shows that there have been great changes in the organisms that have lived on
Earth, and it teaches us three fundamental lessons about life on Earth:
1. Many past organisms were unlike living organisms.
2. Many organisms that were once common are now extinct.
3. New groups of organisms arose from previously existing ones.
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The fossil record can also be used to test our understanding of evolutionary patterns; that is, the
order in which organisms have evolved.
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Fossil fishes occur earlier in the fossil record than all other vertebrates, with amphibians next,
followed by reptiles, then mammals and birds.
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This is consistent with other types of evidence that describe the evolutionary history of
vertebrates.
The fossil record is an incomplete chronicle of evolutionary change.
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The fossil record is biased in favor of species that persisted for a long time, that were abundant and
widespread, that lived in certain environments, and that had hard parts that fossilized readily.
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Few organisms are preserved as fossils. Many fossils that do form are later destroyed by
geologic processes, and only a fraction of them are discovered.
Organisms with hard skeletons (dinosaurs) or coverings (insects) are more likely to fossilize than
soft-bodied organisms.
Despite its limitations, the fossil record provides a detailed account of biological change over
geologic time.
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How fossils “work”
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Sedimentary rocks are the richest source of fossils.
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The relative sequence of fossils in sedimentary rock strata tells
us the order in which the fossils were formed, although it does
not tell us their ages.
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Geologists have developed methods for obtaining absolute
dates for fossils.
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One of the most common techniques is radiometric dating,
which is based on the decay of radioactive isotopes.
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Remember than an isotope is a version of an atom that has
a different number of neutrons than most of the atoms a
given element.
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Every isotope has a characteristic half-life, which is the
number of years it takes for 50% of the original sample to
decay. This length of time is unaffected by temperature,
pressure, or other environmental variables.
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Carbon-14 decays rapidly, with a half-life of 5,730 years.
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If you have 10 grams of carbon-14 today, when you come back 5,730 years from now, 50%
(5 grams) will have lost its extra neutrons and decayed.
Uranium-238 decays more slowly, with a half-life of 4.5 billion years.
Fossils contain isotopes of elements that organisms take up when they were alive.
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For example, the carbon in a living organism contains the most common carbon isotope,
carbon-12, as well as a radioactive isotope, carbon-14.
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When an organism dies, it stops accumulating carbon, and the carbon-14 that it contained at
the time of death slowly decays.
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By measuring the ratio of carbon-14 to total carbon in a fossil, scientists can determine the
fossil’s age.
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Homology
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Further evidence for evolution comes from studying similarities in the characteristics of different
organisms.
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Descent with modification can explain why certain traits in related species are similar even if they
have very different functions.
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Characteristics present in the shared ancestor are modified (by natural selection) in its
descendants over time as they face different environmental conditions.
Homology refers to a similarity shared by two different species due to their common ancestry.
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For example, the forelimbs of human, cats, whales, and bats share the same skeletal structure,
even though the appendages have very different functions.
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These forelimbs are homologous structures that represent variations on the ancestral
forelimb.
Many homologies are only present during embryonic development and are not obvious in adult
organisms.
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For example, all vertebrate embryos (including humans) have a tail, as well as structures called
pharyngeal pouches in their throat at some stage in their development.
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The homologous throat pouches develop into very different adult structures, such as the gills of
fish or parts of the ears and throat in humans and other mammals.
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While we may not think that a fish’s gills and our ears look or act the same, they are
homologous due to their shared embryological origins.
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Some of the most interesting homologous structures are vestigial organs, structures that have
little, if any, importance to a living organism but that had important functions in the organism’s
ancestors.
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For example, the skeletons of some snakes and of fossil whales retain vestiges of the pelvis and
leg bones of walking ancestors.
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Examples of vestigial structures in humans include our tailbones, appendix, and goosebumps.
Checking Your Understanding
☛ How might vestigial structures be explained if we use Lamarck’s theory of the inheritance of acquired
characteristics?
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Similarities among organisms can also be seen at the molecular level.
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For example, all species of life have the same basic genetic machinery of RNA and DNA, and
the genetic code is essentially universal.
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By genetic code, we mean the relationship between codons (e.g., AUG and CGA) and
amino acids (e.g., Met and Arg).
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The use of this genetic code by all living organisms supports the theory of a single origin of all
life.
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It is likely that the language of the genetic code has been passed along through all the branches
of the tree of life ever since its inception in an early life form.
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Organisms as different as humans and bacteria share genes that have been inherited from a
very distant common ancestor.
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Like the forelimbs of humans and whales, these genes have often acquired different
functions over time.
Homologies mirror the tree of life.
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Some homologies, such as the genetic code, are shared by all living things because they arose
in the deep ancestral past.
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Other homologies that evolved more recently are shared by only smaller branches of the tree of
life.
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For example, all tetrapods (four-limbed animals; amphibians, reptiles, birds, and mammals)
share the same limb structure. This is a more modern feature, shared only by a more
closely-related group of organisms.
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Thus homologies are found in a nested pattern, with all life sharing the deepest layer and each
smaller group adding new homologies to those it shares with the larger group.
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This hierarchical pattern of homology is exactly what we would expect if life evolved and
diversified from a common ancestor.
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The pattern of descent from common ancestors and the resulting homologies can be shown in an
evolutionary tree, which reflects evolutionary relationships among groups of organisms.
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Evolutionary trees are hypotheses that summarize our current understanding of patterns of common
descent.
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Some trees are supported by a variety of data, including anatomical and DNA sequence data.
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Scientists can use well-supported evolutionary trees to make predictions about the biology of
organisms.
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Checking Your Understanding
☛ Are crocodiles more closely related to lizards or birds?
☛ Which characteristic is more ancient: feathers or an amnion (that’s a protective membrane that
surrounds a growing embryo)?
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Distantly related organisms may resemble each other because of
convergent evolution, the independent evolution of similar
features.
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For example, many marsupial mammals, such as these sugar
gliders from Australia, resemble specific non-marsupials from
other continents.
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The two types of mammals are similar because of convergent
evolution. Each group independently evolved the ability to glide
from tree to tree.
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Their resemblance is analogous, not homologous.
Biogeography
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Evidence for evolution also comes from the geographic distribution
of species—biogeography.
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The geographic distribution of organisms is influenced by many
factors, including continental drift.
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Two hundred fifty million years ago, all of Earth’s landmasses
joined to form a single large continent called Pangaea.
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Pangaea broke apart 200 million years ago. By 20 million
years ago, the continents were within a few hundred
kilometers of their present locations.
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From our understanding of evolution and continental drift, we can
predict where fossils of various types of organisms might be
found.
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We can use our understanding of evolution to explain
biogeographic data.
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For example, islands generally have endemic species that are found nowhere else on Earth.
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Most island species are closely related to species from the nearest mainland or a neighboring
island, reflecting the pattern of colonization of the island.
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The colonists eventually give rise to new species as they adapt to their environments.
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Two islands with similar environments are populated, not by closely related species, but rather
by species that resemble those of the nearest mainland.
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In the example below, the smaller island fox found on the Channel Islands (b) evolved from the
gray fox found on the California mainland (a).
Checking Your Understanding
☛ The fossil record show that dinosaurs originated on Earth 200-250 million years ago. Would you
expect the geographic distribution of early dinosaur fossils to be broad (on many continents) or narrow
(on one or a few continents only)? Explain.
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Review Questions
Testing Your Knowledge
☛ Textbook Self-Quiz Questions 2, 4, 5, and 6 (p. 467)
☛ How do scientists date fossils?
☛ Explain how the homologous forelimbs in vertebrates support the idea of biological evolution.
☛ Define convergent evolution and give an example.
Testing Your Understanding
☛ How does developmental biology provide evidence of common ancestry for vertebrates as diverse as
reptiles, birds, pigs, and humans?
Applying Concepts to New Situations
☛ How can we explain that fossils of Mesosaurus, an extinct reptile that could not swim across open
water, are found in southern parts of both Africa and South America?
Recommended Textbook Website Activities
Reconstructing Forelimbs
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