CH21 – Mechanisms of Evolution
21.1 What Is the Relationship between Fact and Theory in Evolution?
change in the genetic composition of populations over time is called evolution
In the fossil record, we observe the long-term morphological changes that are the
result of underlying genetic changes.
The resulting understanding of the mechanisms of evolutionary change is known as
evolutionary theory.
At a more basic level, evolutionary theory allows biologists to understand how life
diversifies and how species interact. It also helps us make predictions about the
biological world.
It is evolutionary theory, however, that allows us to apply our understanding of
evolution to problems in medicine, agriculture, industry, and throughout biology
--Darwin and Wallace introduced the idea of evolution by natural selection:
Darwin developed the framework of an explanatory theory for evolutionary change based on
three major propositions:
• Species are not immutable; they change over time.
• Divergent species share a common ancestor and have diverged from one another gradually
through time (a concept Darwin termed descent with modification).
• Changes in species over time can be explained by natural selection: the differential survival
and reproduction of individuals based on variation in their traits.
--Evolutionary theory has continued to develop over the past century
--Genetic variation contributes to phenotypic variation:
Phenotypes the physical expressions of organisms’ genes (including interactions among
those genes).
The features of a phenotype are its characters—eye color, for example.
The specific form of a character, such as brown eyes, is a trait.
A heritable trait is a trait that is at least partly determined by the organism’s genes.
The genetic constitution that governs a character is called its genotype. A population evolves
when individuals with different genotypes survive or reproduce at different rates.
Different forms of a gene, known as alleles, may exist at a locus (a particular site on a
chromosome).
At any given locus, a single diploid individual carries no more than two of all the alleles
found in the population. The sum of all copies of all alleles at all loci found in a population
constitutes that population’s gene pool. The gene pool contains the genetic variation that
produces the phenotypic traits on which natural selection acts.
Evolution can be defined as changes in the proportions of alleles in the gene pool over time.
,genotypes alone do not determine all phenotypes. When one allele is dominant to another, for
example, a particular phenotype can be produced by more than one genotype. (e.g., AA and
Aa individuals may be phenotypically identical).
Evolutionary change is directly observable in biological populations. Natural selection is one
of the major mechanisms that results in evolution. It acts on genetic variation, which is
required for evolutionary change to occur.
21.2 What Are the Mechanisms of Evolutionary Change?
Evolution is genetic change occurring in a population—a group of individuals of a single
species that live and interbreed in a particular geographic area. It is important to remember
that individuals do not evolve; populations do.
Five processes— natural selection, mutation, gene flow, genetic drift, and nonrandom
mating—affect the genetic makeup of populations over time and thus can result in evolution.
1- Mutation generates genetic variation:
a mutation is any change in the nucleotide sequence of an organism’s DNA.
it is natural selection acting on this random variation that results in adaptation. Most
mutations are either harmful to their bearers (deleterious mutations) or have no effect (neutral
mutations). mutation both creates and helps maintain genetic variation in populations.
Mutation rates can be very high, particularly in viruses. Even though the mutation rate in
humans is low, human populations still contain enormous genetic variation on which other
evolutionary mechanisms can act.
Mutation adds new alleles to the gene pool. Biologists use two simple measures to
characterize the variation in a given gene pool. The proportion of each allele in the gene pool
is its allele frequency. Similarly, the proportion of each genotype among the individuals in
the population is its genotype frequency. The calculations of allele and genotype frequencies
in a population allow biologists to measure evolutionary change
2- Selection acting on genetic variation leads to new phenotypes:
artificial selection—the purposeful selection of specific phenotypes by humans—e.g., on
different characters in a single European species of wild mustard has resulted in many
different crop plants.
Natural selection resulted in traits that helped organisms survive and reproduce more
effectively; artificial selection resulted in traits that were preferred by the human breeders, for
whatever reason.
A favored trait that evolves through natural selection is known as an adaptation
One consequence of natural seection is the purging of deleterious mutations from populations.
Individuals with deleterious mutations are less likely to survive and reproduce, so they are
less likely to pass their alleles on to the next generation. Biologists often distinguish between
two broad categories of selection: positive selection (selection for beneficial changes) and
purifying selection (selection against deleterious changes).
3- Gene flow may change allele frequencies:
,Migration of individuals and movements of gametes (in pollen, for example) between
populations—a phenomenon called gene flow—can change allele frequencies in a population.
If the arriving individuals survive and reproduce in their new location, they may add new
alleles to the population’s gene pool, or they may change the frequencies of alleles present in
the original population.
This incorporation of Neanderthal genes is an example of gene flow. Traits such as red hair
(which was common in Neanderthal populations) may have entered modern human
populations in this manner.
4- Genetic drift may cause large changes in small populations:
In small populations, genetic drift—random changes in allele frequencies from one generation
to the next—may produce large changes in allele frequencies over time. Harmful alleles may
increase in frequency, and rare advantageous alleles may be lost. Even in large populations,
genetic drift can influence the frequencies of neutral alleles (which do not affect the survival
and reproductive rates of their bearers).
Genetic drift is particularly potent when a population is reduced dramatically in size. Even
populations that are normally large may occasionally pass through environmental conditions
that only a small number of individuals survive, a situation known as a population
bottleneck. Population bottlenecks occur when only a few individuals survive a random
event. The result may be a change in allele frequencies within the population.
A population forced through a bottleneck is likely to lose much of its genetic variation.
Genetic drift can have similar effects when a few pioneering individuals colonize a new
region. Because of its small size, the colonizing population is unlikely to possess all the
alleles found in the gene pool of its source population. The resulting reduction in genetic
variation, called a founder effect, is equivalent to that in a large population reduced by a
bottleneck.
5- Nonrandom mating can change genotype or allele frequencies
Mating patterns often alter genotype frequencies because the individuals in a population do
not choose mates at random.
Nonrandom mating systems that do not affect the relative reproductive success of individuals
produce changes in genotype frequencies but not in allele frequencies, and thus do not, by
themselves, result in evolutionary change in a population. However, nonrandom mating
systems that result in differential reproductive success among individuals do produce allele
frequency changes from one generation to the next. Sexual selection occurs when individuals
of one sex mate preferentially with particular individuals of the opposite sex rather than at
random. these features either improved the ability of their bearers to compete for access to
mates (intrasexual selection) or made their bearers more attractive to members of the opposite
sex (intersexual selection).
Darwin argued that while natural selection typically favors traits that enhance the survival of
their bearers or their descendants, sexual selection is primarily about successful reproduction
sexual selection may favor traits that enhance an individual’s chances of reproduction even if
they reduce its chances of survival. For example, females may be more likely to see or hear
, males with a conspicuous trait (and thus be more likely to mate with those males), even
though the conspicuous trait may increase the chances that the male will be seen or heard by a
predator. The sexual signal may also indicate a successful genotype in the male.
Males with artificially elongated tails attracted about four times more females than did males
with shortened tails. the reasons why females prefer sexually selected traits are not always
clear. What is clear is that female preferences often lead to the evolution of dramatic male
adornments and mating displays.
21.3 How Do Biologists Measure Evolutionary Change?
we can measure evolution by looking at gradual changes in allele and genotype frequencies
(or of their respective phenotypes) in populations.
1- Evolutionary change can be measured by allele and genotype frequencies:
An allele’s frequency is calculated using the following formula:
p =number of copies of the allele in the population/ total number of copies of all alleles in the
population
If only two alleles (we’ll call them A and a) are found among the members of a diploid
population, those alleles can combine to form three different genotypes: AA, Aa, and aa. A
population with more than one allele at a locus is said to be polymorphic (“many forms”) at
that locus.
the total number of individuals in the
population, and that the total number of copies
of both alleles present in the population is 2N,
because each individual is diploid.
If there is only one allele at a given locus in a
population, its frequency is 1: The population
is then monomorphic (“one form”) at that
locus, and the allele is said to be fixed.
The frequencies of the different alleles at each
locus and the frequencies of the different
genotypes in a population describe that
population’s genetic structure.
Allele frequencies measure the amount of
genetic variation in a population; genotype
frequencies show how a population’s genetic
variation is distributed among its members.
2- Evolution will occur unless certain restrictive conditions exist
Hardy–Weinberg equilibrium constitutes a model in which allele frequencies do not change
across generations, and genotype frequencies can be predicted from allele frequencies. The
expectations of Hardy– Weinberg equilibrium apply only to sexually reproducing organisms.
Five mechanisms of evolution:
21.1 What Is the Relationship between Fact and Theory in Evolution?
change in the genetic composition of populations over time is called evolution
In the fossil record, we observe the long-term morphological changes that are the
result of underlying genetic changes.
The resulting understanding of the mechanisms of evolutionary change is known as
evolutionary theory.
At a more basic level, evolutionary theory allows biologists to understand how life
diversifies and how species interact. It also helps us make predictions about the
biological world.
It is evolutionary theory, however, that allows us to apply our understanding of
evolution to problems in medicine, agriculture, industry, and throughout biology
--Darwin and Wallace introduced the idea of evolution by natural selection:
Darwin developed the framework of an explanatory theory for evolutionary change based on
three major propositions:
• Species are not immutable; they change over time.
• Divergent species share a common ancestor and have diverged from one another gradually
through time (a concept Darwin termed descent with modification).
• Changes in species over time can be explained by natural selection: the differential survival
and reproduction of individuals based on variation in their traits.
--Evolutionary theory has continued to develop over the past century
--Genetic variation contributes to phenotypic variation:
Phenotypes the physical expressions of organisms’ genes (including interactions among
those genes).
The features of a phenotype are its characters—eye color, for example.
The specific form of a character, such as brown eyes, is a trait.
A heritable trait is a trait that is at least partly determined by the organism’s genes.
The genetic constitution that governs a character is called its genotype. A population evolves
when individuals with different genotypes survive or reproduce at different rates.
Different forms of a gene, known as alleles, may exist at a locus (a particular site on a
chromosome).
At any given locus, a single diploid individual carries no more than two of all the alleles
found in the population. The sum of all copies of all alleles at all loci found in a population
constitutes that population’s gene pool. The gene pool contains the genetic variation that
produces the phenotypic traits on which natural selection acts.
Evolution can be defined as changes in the proportions of alleles in the gene pool over time.
,genotypes alone do not determine all phenotypes. When one allele is dominant to another, for
example, a particular phenotype can be produced by more than one genotype. (e.g., AA and
Aa individuals may be phenotypically identical).
Evolutionary change is directly observable in biological populations. Natural selection is one
of the major mechanisms that results in evolution. It acts on genetic variation, which is
required for evolutionary change to occur.
21.2 What Are the Mechanisms of Evolutionary Change?
Evolution is genetic change occurring in a population—a group of individuals of a single
species that live and interbreed in a particular geographic area. It is important to remember
that individuals do not evolve; populations do.
Five processes— natural selection, mutation, gene flow, genetic drift, and nonrandom
mating—affect the genetic makeup of populations over time and thus can result in evolution.
1- Mutation generates genetic variation:
a mutation is any change in the nucleotide sequence of an organism’s DNA.
it is natural selection acting on this random variation that results in adaptation. Most
mutations are either harmful to their bearers (deleterious mutations) or have no effect (neutral
mutations). mutation both creates and helps maintain genetic variation in populations.
Mutation rates can be very high, particularly in viruses. Even though the mutation rate in
humans is low, human populations still contain enormous genetic variation on which other
evolutionary mechanisms can act.
Mutation adds new alleles to the gene pool. Biologists use two simple measures to
characterize the variation in a given gene pool. The proportion of each allele in the gene pool
is its allele frequency. Similarly, the proportion of each genotype among the individuals in
the population is its genotype frequency. The calculations of allele and genotype frequencies
in a population allow biologists to measure evolutionary change
2- Selection acting on genetic variation leads to new phenotypes:
artificial selection—the purposeful selection of specific phenotypes by humans—e.g., on
different characters in a single European species of wild mustard has resulted in many
different crop plants.
Natural selection resulted in traits that helped organisms survive and reproduce more
effectively; artificial selection resulted in traits that were preferred by the human breeders, for
whatever reason.
A favored trait that evolves through natural selection is known as an adaptation
One consequence of natural seection is the purging of deleterious mutations from populations.
Individuals with deleterious mutations are less likely to survive and reproduce, so they are
less likely to pass their alleles on to the next generation. Biologists often distinguish between
two broad categories of selection: positive selection (selection for beneficial changes) and
purifying selection (selection against deleterious changes).
3- Gene flow may change allele frequencies:
,Migration of individuals and movements of gametes (in pollen, for example) between
populations—a phenomenon called gene flow—can change allele frequencies in a population.
If the arriving individuals survive and reproduce in their new location, they may add new
alleles to the population’s gene pool, or they may change the frequencies of alleles present in
the original population.
This incorporation of Neanderthal genes is an example of gene flow. Traits such as red hair
(which was common in Neanderthal populations) may have entered modern human
populations in this manner.
4- Genetic drift may cause large changes in small populations:
In small populations, genetic drift—random changes in allele frequencies from one generation
to the next—may produce large changes in allele frequencies over time. Harmful alleles may
increase in frequency, and rare advantageous alleles may be lost. Even in large populations,
genetic drift can influence the frequencies of neutral alleles (which do not affect the survival
and reproductive rates of their bearers).
Genetic drift is particularly potent when a population is reduced dramatically in size. Even
populations that are normally large may occasionally pass through environmental conditions
that only a small number of individuals survive, a situation known as a population
bottleneck. Population bottlenecks occur when only a few individuals survive a random
event. The result may be a change in allele frequencies within the population.
A population forced through a bottleneck is likely to lose much of its genetic variation.
Genetic drift can have similar effects when a few pioneering individuals colonize a new
region. Because of its small size, the colonizing population is unlikely to possess all the
alleles found in the gene pool of its source population. The resulting reduction in genetic
variation, called a founder effect, is equivalent to that in a large population reduced by a
bottleneck.
5- Nonrandom mating can change genotype or allele frequencies
Mating patterns often alter genotype frequencies because the individuals in a population do
not choose mates at random.
Nonrandom mating systems that do not affect the relative reproductive success of individuals
produce changes in genotype frequencies but not in allele frequencies, and thus do not, by
themselves, result in evolutionary change in a population. However, nonrandom mating
systems that result in differential reproductive success among individuals do produce allele
frequency changes from one generation to the next. Sexual selection occurs when individuals
of one sex mate preferentially with particular individuals of the opposite sex rather than at
random. these features either improved the ability of their bearers to compete for access to
mates (intrasexual selection) or made their bearers more attractive to members of the opposite
sex (intersexual selection).
Darwin argued that while natural selection typically favors traits that enhance the survival of
their bearers or their descendants, sexual selection is primarily about successful reproduction
sexual selection may favor traits that enhance an individual’s chances of reproduction even if
they reduce its chances of survival. For example, females may be more likely to see or hear
, males with a conspicuous trait (and thus be more likely to mate with those males), even
though the conspicuous trait may increase the chances that the male will be seen or heard by a
predator. The sexual signal may also indicate a successful genotype in the male.
Males with artificially elongated tails attracted about four times more females than did males
with shortened tails. the reasons why females prefer sexually selected traits are not always
clear. What is clear is that female preferences often lead to the evolution of dramatic male
adornments and mating displays.
21.3 How Do Biologists Measure Evolutionary Change?
we can measure evolution by looking at gradual changes in allele and genotype frequencies
(or of their respective phenotypes) in populations.
1- Evolutionary change can be measured by allele and genotype frequencies:
An allele’s frequency is calculated using the following formula:
p =number of copies of the allele in the population/ total number of copies of all alleles in the
population
If only two alleles (we’ll call them A and a) are found among the members of a diploid
population, those alleles can combine to form three different genotypes: AA, Aa, and aa. A
population with more than one allele at a locus is said to be polymorphic (“many forms”) at
that locus.
the total number of individuals in the
population, and that the total number of copies
of both alleles present in the population is 2N,
because each individual is diploid.
If there is only one allele at a given locus in a
population, its frequency is 1: The population
is then monomorphic (“one form”) at that
locus, and the allele is said to be fixed.
The frequencies of the different alleles at each
locus and the frequencies of the different
genotypes in a population describe that
population’s genetic structure.
Allele frequencies measure the amount of
genetic variation in a population; genotype
frequencies show how a population’s genetic
variation is distributed among its members.
2- Evolution will occur unless certain restrictive conditions exist
Hardy–Weinberg equilibrium constitutes a model in which allele frequencies do not change
across generations, and genotype frequencies can be predicted from allele frequencies. The
expectations of Hardy– Weinberg equilibrium apply only to sexually reproducing organisms.
Five mechanisms of evolution:
