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Date
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15.1 Natural Selection and the
Evidence for Evolution
Indiana Standards Standard B.1.32 Explain how natural selection leads to
◗ Before You Read
organisms that are well suited for survival in particular environments, and discuss
how natural selection provides scientific explanation for the history of life on Earth
as depicted in the fossil record and in the similarities evident within the diversity of
existing organisms.
In biology, evolution means that populations of a species change over time. In this section you will
learn about Charles Darwin and his theory of evolution. Skim the Read to Learn section below and
find three important facts about Darwin. Write those facts on the lines below. After you have written
your facts, highlight the one you think is the most important.
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Charles Darwin and Natural Selection
Evolution describes the way populations change over time. The
modern theory of evolution, in fact, is the main concept in biology. What you learn about evolution will make it easier for you to
understand the subject of biology. A place to start is by learning
about the ideas of Charles Darwin, an English naturalist who
lived from 1809 to 1882. Darwin’s ideas about evolution have
been supported by fossil evidence.
How did fossils shape ideas about evolution?
A fossil is evidence that an organism lived long ago. Scientists
wondered how fossils formed. They wondered why many fossil
species had died out or become extinct. They also wanted to
know more about how extinct species and modern species might
be related. There were many ideas about how species evolved.
But the ideas of Charles Darwin became the basis of modern
evolutionary theory.
Locate
Information Underline every
heading in the reading that
asks a question. Then, use a
different color to highlight
the answers to those questions as you find them.
1. What forms the basis of
modern evolutionary
theory?
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What did Darwin study?
Darwin spent five years on a research voyage around the world.
He became interested in how species might be related to one
another. While in the Galápagos (guh LAH puh gus) Islands off
the west coast of South America, Darwin saw many species of
plants and animals. He noted that these species looked similar to
species he had seen in other places. He wondered if a species
might be able to change over time. But at the time, he could not
explain how such changes might happen.
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Chapter 15
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Section
15.1
Natural Selection and the Evidence for Evolution, continued
After returning to England, Darwin spent twenty years doing
research. He studied, experimented, read, and collected samples.
He tried to figure out why some animals survive and others do
not. Darwin bred pigeons and saw that there were small differences, or variations, in traits of individual pigeons. He also noticed
that these traits could be inherited by offspring. Eventually, he
conducted an experiment where he bred pigeons that had certain
desirable traits. He observed that their offspring were born with
the same desirable traits. Breeding organisms with a certain
trait to produce offspring with identical traits is called artificial
selection. Darwin decided that there must be a process in the
natural world that works like artificial selection. Using evidence
from his research, Darwin decided that that process in nature was
natural selection.
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In natural selection, organisms with favorable traits are able
to reproduce and pass their traits on to their offspring, who then
reproduce. Those without such favorable traits are more likely to
die out before reproducing. For example, suppose fish that are
slow get eaten before they can reproduce, while fish that are fast
survive and reproduce. These offspring inherit the trait of speed
from their parents. This way, they too are more likely to survive
and pass on that trait to their offspring.
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What have we learned since Darwin?
Much evidence supports Darwin’s theories. However, it is
hard to directly observe evolutionary processes that take place
over millions of years. Despite this, much data has been gathered
for many years from many sources. Most of today’s biologists
agree that evolution by natural selection best explains this data.
The study of genetics adds even more to our understanding of
evolution. We now know that traits are controlled by genes. All
the genes that are available in a population are its gene pool.
Changes in a population’s gene pool over time play an important
role in evolution.
Adaptations: Evidence for Evolution
An adaptation is anything that gives an organism a better chance
of survival. The two main types of adaptations are structural
adaptations and physiological (fih zee uh LAH jih kul) adaptations.
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What is natural selection?
2. What is the process in
which organisms with
favorable traits tend to
survive and pass on these
traits to their offspring?
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Date
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15.1
Natural Selection and the Evidence for Evolution, continued
What are structural adaptations?
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Structural adaptations take many different forms. Thorns, teeth,
hair, beaks, and color are examples of structural adaptations that
are inherited. Some adaptations take millions of years to become
widespread in a population. Mole rats developed large teeth and
claws. This structural adaptation helps them dig holes and protect
themselves. Adaptations that keep predators from approaching an
organism include a rose’s thorns and a porcupine’s quills.
Some animals develop coloring that helps them blend with their
surroundings. This is an example of a subtle structural adaptation
called camouflage (KA muh flahj). Camouflaged organisms survive
and reproduce because they cannot be easily found by predators.
Mimicry (MIHM ih kree) is another type of structural adaptation. It occurs when one species looks like another species. In one
form of mimicry, a harmless species takes on the look of a dangerous species. Predators that avoid the harmful species have a hard
time telling the two species apart, and so they avoid both. In this
way, the harmless species benefits. Another type of mimicry happens when two or more harmful species grow to resemble each
other. For example, bees, wasps, and yellow jacket hornets all look
alike and can sting. For this reason, some predators stay away
from anything that has a bee-like appearance.
3. Analyze Which of the
following is an example
of mimicry? (Circle your
choice.)
a. A harmless fly looks
like a wasp.
b. A frog’s color matches
the tree it lives in.
c. A pesticide stops
working on certain
types of weeds.
What are physiological adaptations?
Some changes in gene pools can happen fairly quickly. A few
medicines that have been developed within the last 50 years have
begun to lose their effectiveness. The bacteria that the medicines
used to treat have undergone physiological adaptations. These
adaptations keep the bacteria from being killed off by various medications. Physiological adaptations are changes in an organism’s
metabolic processes. Some insects and weeds also have evolved to
the point where they are not affected by chemical sprays.
Other Evidence for Evolution
Structural and physiological adaptations are considered direct
evidence of evolution. But most of the evidence to support evolution is indirect. It comes from fossils and sciences such as anatomy,
embryology (em bree AHL uh jee), and biochemistry. Scientists
do not have fossils for all the changes that have taken place. However, fossils provide a big picture of how groups have changed.
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Natural Selection and the Evidence for Evolution, continued
Age
Paleocene
65 million
years ago
Eocene
54 million
years ago
Oligocene
33 million
years ago
Miocene
23 million
years ago
Present
Organism
Skull and
teeth
Fossils are important to the study of evolution because they
provide a record of early life. When you compare an organism as
it looks today with a fossil of that organism, you can see how it
has changed over time. For example, scientists have learned from
fossils that the ancestors of camels were as small as rabbits are
today. This is illustrated on the table above.
4. What is the term for
structural features that
have a common evolutionary origin?
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What can anatomy teach us about evolution?
Homologous Structures The anatomy of different organisms
also shows evolutionary patterns. For example, some organisms
have homologous structures. These are structural features
with a common evolutionary origin. Such structures can be similar in arrangement, function, or both.
The figure on page 167 shows how the forelimbs of three very
different animals can be homologous. Biologists think that such
similarities are evidence that these organisms evolved from a
common ancestor.
Analogous Structures However, being structurally similar does
not always mean that two species are closely related. For instance,
birds and butterflies both have wings. But insects and birds
evolved separately. When body parts of organisms do not have a
common evolutionary origin but are similar in function, they are
known as analogous structures.
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Limb bones
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Natural Selection and the Evidence for Evolution, continued
Crocodile
forelimb
Whale
forelimb
Bird
wing
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Although analogous structures do not have the same origin,
they do provide evidence of evolution. For example, the ancestors
of birds and insects both probably evolved wings separately while
adapting to similar ways of life.
Vestigial Structures Another type of body feature that shows evolutionary relationship is a vestigial (veh STIH jee ul) structure.
This is a body structure in a present-day organism that no longer
serves its original purpose. The eyes of mole rats are an example.
Mole rats still have eyes, but they are no longer used for sight.
Vestigial structures are evidence of evolution because they show
structural change over time.
Embryology An embryo is the earliest stage of growth and
development of a plant or animal. Young embryos of fishes, birds,
reptiles, and mammals have structures that suggest they all had a
common ancestor.
5. Compare What is the
difference between
analogous and vestigial
structures?
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What evidence does biochemistry provide for
evolution?
Biochemistry also provides strong evidence for evolution. Nearly
all organisms share DNA, ATP, and many enzymes in their chemical makeup. Groups that share more similarities in their biochemistry are considered to be more closely related. In the 1970s,
biologists began to use RNA and DNA nucleotide sequences to
construct evolutionary diagrams that show the levels of relationship among species. Today, scientists combine data from fossils
and studies of anatomy, embryology, and biochemistry to interpret relationships among species.
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Natural Selection and the Evidence for Evolution, continued
◗ After You Read
Mini Glossary
analogous structures: structures that do not
have a common evolutionary origin but are
similar in function
artificial selection: process of breeding organisms with specific traits to produce offspring
with the same traits
camouflage (KA muh flahj): structural adaptation that enables species to blend with their
surroundings; allows a species to avoid
detection by predators
embryo: the earliest stage of growth and development of a plant or an animal
homologous structures: structures with common
evolutionary origin; can be similar in
arrangement, function, or both
mimicry: structural adaptation that enables one
species to resemble another species; may
provide protection from predators or other
advantages
natural selection: mechanism for change in
populations; occurs in nature when organisms with favorable variations survive,
reproduce, and pass their variations to the
next generation
vestigial (veh STIH jee ul) structure: a structure
in a present-day organism that no longer
serves its original purpose, but was probably
useful to its ancestor
1. Read the terms and their definitions in the Mini Glossary above. Then, choose a term that
describes a type of structural adaptation. On the lines below, write a sentence using the term.
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2. Use the table below to review what you have learned about adaptation. Write two types of
adaptations you read about in the first column. Write a fact about the rate at which the adaptation occurs in the second column, and an example of each type of adaptation in the third.
Adaptation Table
Type of Adaptation
Rate at Which it Occurs
Example
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Section
15.2 Mechanisms of Evolution
Indiana Standards Standard B.1.36 Trace the relationship between environmental changes and changes in the
gene pool, such as genetic drift and isolation of sub-populations.
◗ Before You Read
In this section you will learn about different ways that evolution occurs. For example, evolution can
occur when a physical barrier divides a population into smaller groups that can no longer interact.
What barriers might divide a population? Write examples on the lines below.
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Population Genetics and Evolution
In the previous section you learned about Darwin’s theory of
evolution by natural selection. Since Darwin’s time, scientists have
learned about genes and have modified Darwin’s theories. Today,
scientists look at the way genes act in plant and animal populations. This study is called population genetics. It is based on the
thought that evolution happens when the genes in a population
change over a long period of time.
Individuals in a population do not evolve—populations do.
Individuals’ genes do not change during their lifetime. But within
a population, genes and their frequencies change over time. This
is what causes evolution.
Identify Main
Ideas Highlight the main
idea of each paragraph.
What is genetic equilibrium?
How can a population’s genes change over time? Picture all the
alleles (the alternate forms of a gene) of a population’s genes in a
large group called a gene pool. The percentage of times any
allele is in the gene pool is called allelic frequency. When this
frequency stays the same over generations, genetic equilibrium
exists. A population in genetic equilibrium is not evolving. Once a
change happens, though, the population’s genetic equilibrium is
disrupted and evolution takes place.
What can change genetic equilibrium?
One way genetic equilibrium is disturbed is by mutation. A
mutation is any change or random error in a DNA sequence.
Some mutations simply occur by chance. Radiation and chemicals
can also cause mutation.
READING ESSENTIALS
1. Analyze Which of the
following is an example
of genetic equilibrium?
(Circle your choice.)
a. Generation after generation of a population of roses are red.
b. A mutation in a population of red roses
results in some yellow
offspring.
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Mechanisms of Evolution, continued
2. Analyze On what population would genetic drift
most likely have the
greatest impact? (Circle
your choice.)
a. a population of twelve
turtles on a small, isolated island
b. the population of
humans in the United
States
c. a population of 5000
woodpeckers in western Canada
Genetic drift is another way that a population’s genetic equilibrium can be disrupted. Genetic drift is the change of allelic
frequencies by chance events. This change can greatly affect
small populations made up of descendants of a small number
of organisms.
For example, in Pennsylvania, there is a small Amish population
of about 12 000 people. The Amish marry only other members of
their community. Of the original 30 settlers in this community, at
least one carried a recessive allele that resulted in offspring with
short arms and legs and extra fingers and toes. Today, the frequency of this allele in this population is high—1 in 14. But, in the
rest of the United States, the frequency is lower, only 1 in 1000.
Gene flow also can upset genetic equilibrium. Gene flow
occurs when an individual leaves or enters a population. This
individual’s genes either leave or enter the gene pool as a result.
Mutation, genetic drift, and gene flow primarily affect small
and isolated gene pools. The impact is much smaller in larger, less
isolated gene pools.
How do changes in a gene pool bring about evolution of a
new species? Remember that a species is a group of living things
that look alike and can mate with each other to produce fertile
offspring. The evolution of a new species is called speciation
(spee shee AY shun). Speciation occurs when members of similar
populations no longer mate with each other to produce fertile offspring. A new species could develop when part of a population
has been geographically cut off from the rest of its population.
The figure on page 171 illustrates this idea.
An example would be when a river forms a physical barrier and
divides a population in two. This is called geographic isolation.
The separated parts of the population can no longer mate. Over
time, the gene pools of the now separate populations become very
different. In this way, natural selection results in new species.
3. What is one way that
reproductive isolation
can occur?
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What happens in reproductive isolation?
As populations become increasingly different from each other,
reproductive isolation occurs. This happens when organisms
that at one time mated with each other and produced fertile offspring are no longer able to do so. This can be because the genetic
material of the populations becomes so different that fertilization
cannot occur. Reproductive isolation also occurs if the mating
seasons of similar populations are at different times of year.
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The Evolution of the Species
Name
Date
Class
Section
15.2
Mechanisms of Evolution, continued
A Tree frogs are a
Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
single population.
B
The formation of a river may
divide the frogs into two
populations. A new form may
appear in one population.
C Over time, the divided
populations may become two
species that may no longer
interbreed, even if reunited.
What role do chromosomes play in the
development of a new species?
Chromosomes can also be important in the development of
new species. Many new types of plants and some types of animals
evolve as a result of what is called polyploidy (PAH lih ploy dee).
Any individual or species with a multiple (an extra set) of the normal set of chromosomes is known as a polyploid. Mistakes during
mitosis or meiosis result in polyploid individuals. Some polyploids
cannot produce offspring capable of reproducing. But still others
develop into adults that can interbreed and a new species results.
Many flowering plants and some important crops—such as wheat,
cotton, and apples—originated by polyploidy.
How much time does it take to develop a
new species?
Although the developing of new species by polyploidy takes
only one generation, most other types of speciation take much
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Mechanisms of Evolution, continued
longer. Gradualism is the idea that species originate through a
gradual change of adaptations. For example, fossil evidence shows
that sea lilies evolved slowly and steadily over time.
Punctuated equilibrium is a theory that speciation occurs
quickly, in rapid bursts. There are long periods of genetic equilibrium between the bursts. In this theory, environmental changes
like higher temperatures or a competitive species moving into a
population’s habitat lead to fast changes in the population’s gene
pool. Fossil evidence shows several elephant species may have
evolved by punctuated equilibrium.
Patterns of Evolution
Extinct
mamo
Amakihi
Possible
ancestral
Lasan finch
Crested
honeycreeper
Kauai
Niihau
Molokai
Oahu
Maui
Lanai
Akialoa
Kahoolawe
Akepa
Akiapolaau
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Apapane
Maui
parrotbill
Liwi
Akikiki
Hawaii
Grosbeak
finch
Palila
Ou
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Biologists have observed that different patterns of evolution
occur in different environments. These patterns support the idea
that natural selection is important for evolution. An example of
this occurs in the Hawaiian honeycreepers.
Hawaiian honeycreepers are all similar in body size and shape,
but they differ in color and beak shape. They also live in different
habitats. Despite their differences, scientists hypothesize that
these birds evolved from a single species that lived on the
Name
Date
Class
Section
15.2
Mechanisms of Evolution, continued
Hawaiian Islands long ago. When a single ancestral species
evolves into many different species that fit a number of different
habitats, the result is called adaptive radiation. Adaptive radiation
is a type of divergent evolution. Divergent evolution occurs as
populations that were once similar to an ancestral species change
and adapt to different living conditions. These populations eventually become new species.
Another pattern that can occur is convergent evolution.
Convergent evolution occurs when unrelated species live in similar environments in different parts of the world. Because they
have similar environmental pressures, they share similar pressures
of natural selection. As a result, they have similarities. For example, there is an organ pipe cactus that grows in the deserts of
North and South America and a plant that looks similar and lives
in African deserts. These plants are not related, but their environments are similar. Both plants have fleshy bodies and no leaves.
Convergent evolution has apparently occurred.
4. What features do
unrelated species that
develop similar traits in
different parts of the
world demonstrate?
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Mini Glossary
adaptive radiation: divergent evolution in which
ancestral species evolve into a variety of
species that fit diverse habitats
allelic frequency: percentage of any specific
allele found in a population’s gene pool
convergent evolution: evolution in which unrelated organisms evolve similar traits; occurs
when unrelated species occupy similar
environments
divergent evolution: evolution in which species
that once were similar to an ancestral
species diverge; occurs when populations
change as they adapt to different environmental conditions, eventually resulting in
new species
gene pool: all of the alleles available in a
population
genetic drift: alteration of allelic frequencies in a
population by chance events; disrupts a
population’s genetic equilibrium
genetic equilibrium: condition in which the frequency of alleles in a population remains the
same over generations; no evolution occurs
READING ESSENTIALS
geographic isolation: occurs whenever a physical
barrier such as a river divides a population;
results in individuals of the population no
longer being able to mate; can lead to the
formation of new species
gradualism: idea that species originate through
a gradual change of adaptations
polyploid: any species with multiple sets of the
normal set of chromosomes; results from
errors during mitosis or meiosis
punctuated equilibrium: idea that periods of
speciation occur relatively quickly with long
periods of genetic equilibrium between
reproductive isolation: occurs when formerly
interbreeding organisms can no longer produce fertile offspring due to an incompatibility of their genetic material or by
differences in mating behavior
speciation (spee shee AY shun): process of evolution of new species that occurs when
members of similar populations no longer
interbreed to produce fertile offspring
within their natural environment
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Section
15.2
Mechanisms of Evolution, continued
1. Read the terms and their definitions from the Mini Glossary on page 173. Circle the two
terms that refer to different ideas about the rate in which speciation occurs. Then, choose
one of these terms and use it correctly in a sentence.
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2. Use the statements under the diagram to fill in the results of each type of isolation.
The Evolution of a New Species
Cause
Effect
Geographic Isolation
Reproductive
Isolation
• Members of the same species no longer mate because they cannot reach each other.
• The genetic material of the populations becomes so different that fertilization cannot occur.
• A barrier divides a population.
3. Review the section, then fill in the blanks below using the following terms: punctuated
equilibrium, speciation, genetic equilibrium, polyploid, mutation.
1. When the frequency of alleles for a specific trait remains the same for generations, it is
called
.
2. One of the factors that can interrupt genetic equilibrium is
3.
.
occurs when members of similar populations can no longer interbreed to produce fertile offspring within their natural environment.
4. The idea that a sudden environmental change can cause rapid changes in a population’s
gene pool is called
5. A
.
is an individual with a multiple of the normal set of chromosomes.
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• Populations have different mating seasons.