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Chapter 23
The Evolution of Populations
Introduction
• One misconception is that organisms
evolve, in the Darwinian sense, during their
lifetimes
• Natural selection acts on individuals, but
only populations evolve
• Genetic variations in populations contribute
to evolution
• Consider, for example, a population of medium
ground finches on Daphne Major Island
– During a drought, large-beaked birds were more
likely to crack large seeds and survive
– The finch population evolved by natural
selection
Figure 23.1
Microevolution
• We can define microevolution as
generation-to-generation change in a
population’s frequencies of alleles.
– Changes in the gene frequency from parents
to offspring
– Microevolution occurs even if the frequencies
of alleles are changing for only a single gene
in a population while the others remain
constant.
• Three mechanisms cause allele frequency
change:
– Natural selection
– Genetic drift
– Gene flow
• Only natural selection causes adaptive evolution
Genetic variation makes
evolution possible
• Variation in heritable traits is a prerequisite for
evolution
• Mendel’s work on pea plants provided evidence
of discrete heritable units (genes)
Concept 23.1: Mutation and sexual
reproduction produce genetic variation
• Two processes, mutation and sexual
reproduction, produce the variation in
gene pools that contributes to differences
among individuals
Genetic Variation
• Genetic variation among individuals is caused by
differences in genes or other DNA segments
• Variation in individual genotype leads to variation in
individual phenotype
• Not all phenotypic variation is heritable
• Natural selection can only act on variation with a genetic
component
Variation Within a Population
• Both discrete and quantitative characters
contribute to variation within a population
• Discrete characters can be classified on
an either-or basis
– Purple or white flowers
• Quantitative characters vary along a
continuum within a population
– Height or skin pigmentation
Variation Between Populations
• Most species exhibit geographic variation,
differences between gene pools of separate
populations
• For example, Madeira is home to several
isolated populations of mice
– Chromosomal variation among populations is
due to drift, not natural selection
Sources of Genetic Variation
• New genes and alleles can arise by mutation or
gene duplication
Mutation
• Mutations are changes in the nucleotide
sequence of DNA
• “Mutations are the ultimate source of new
alleles”
• Only mutations in cells that produce
gametes can be passed to offspring
• The effects of point mutations can vary:
– Mutations in noncoding regions of DNA are
often harmless
– Mutations might not affect protein production
because of redundancy in the genetic code
– Mutations that result in a change in protein
production are often harmful
– Mutations that result in a change in protein
production benefit organism
might
Altering Gene Number or Position
• Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
• Duplication of small pieces of DNA increases
genome size and is usually less harmful
• Duplicated genes can take on new functions by
further mutation
• An ancestral odor-detecting gene has been
duplicated many times: humans have 1,000
copies of the gene, mice have 1,300
Rapid Reproduction
• Mutation rates are low in animals and plants
• The average is about one mutation in every
100,000 genes per generation
• Mutation rates are often lower in prokaryotes
and higher in viruses
Sexual Reproduction
• Sexual reproduction can shuffle existing
alleles into new combinations
• In organisms that reproduce sexually,
recombination of alleles is more important
than mutation in producing the genetic
differences that make adaptation possible
The Hardy-Weinberg equation can
be used to test whether a
population is evolving
• The first step in testing whether evolution is
occurring in a population is to clarify what we
mean by a population
Gene Pools and Allele
Frequencies
• A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
• A gene pool consists of all the alleles for all loci
in a population
• A locus is fixed if all individuals in a population
are homozygous for the same allele
Concept 23.2: The Hardy-Weinberg
equation
• Hardy-Weinberg equilibrium describes the
constant frequency of alleles in such a gene
pool
• If p and q represent the relative frequencies
of the only two possible alleles in a
population at a particular locus, then
– p2 + 2pq + q2 = 1
– where p2 and q2 represent the frequencies of
the homozygous genotypes and 2pq
represents the frequency of the heterozygous
genotype
The Hardy-Weinberg Principle
• The Hardy-Weinberg principle describes a
population that is not evolving
• If a population does not meet the criteria of
the Hardy-Weinberg principle, it can be
concluded that the population is evolving
Hardy-Weinberg Equilibrium
• The Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a
population remain constant from generation to
generation
• In a given population where gametes contribute
to the next generation randomly, allele
frequencies will not change
• Mendelian inheritance preserves genetic
variation in a population
Conditions for Hardy-Weinberg
Equilibrium
• The Hardy-Weinberg theorem describes a
hypothetical population that is not evolving
• In real populations, allele and genotype
frequencies do change over time
• The five conditions for nonevolving
populations are rarely met in nature:
– No mutations
– Random mating
– No natural selection
– Extremely large population size
– No gene flow
• Natural populations can evolve at some loci,
while being in Hardy-Weinberg equilibrium at
other loci
Applying the Hardy-Weinberg
Principle
• We can assume the locus that causes
phenylketonuria (PKU) is in Hardy-Weinberg
equilibrium given that:
1. The PKU gene mutation rate is low
2. Mate selection is random with respect to
whether or not an individual is a carrier for the
PKU allele
3. Natural selection can only act on rare
homozygous individuals who do not follow
dietary restrictions
4. The population is large
5. Migration has no effect as many other
populations have similar allele frequencies
• The occurrence of PKU is 1 per 10,000 births
– q2  0.0001
– q  0.01
• The frequency of normal alleles is
p + q  1; if q = 0.01, then p  0.99
• The frequency of carriers is
– 2pq  2  0.99  0.01  0.0198
– or approximately 2% of the U.S. population
Concept 23.3: Processes that alter
allele frequencies in a population
• Three major factors alter allele frequencies
and bring about most evolutionary change:
– Natural selection*
– Genetic drift
– Gene flow
• *Natural selection is the only factor that
generally adapts a population to its environment.
– Selection always favors the disproportionate
propagation of favorable traits.
Natural Selection
• Natural selection will lead some individuals
to leave more offspring than others
– Selection results in some alleles being passed
along to the next generation disproportionate to
their frequencies in the present generation
• Ultimately natural selection leads to:
– Differential survival
– Differential reproduction
• Natural selection accumulates and maintains
favorable genotypes in a population
The Key Role of Natural
Selection in Adaptive Evolution
• Striking adaptations have arisen by natural
selection
– For example, cuttlefish can change color
rapidly for camouflage
– For example, the jaws of snakes allow them
to swallow prey larger than their heads
Figure 23.14
Bones shown in
green are movable.
Ligament
• Natural selection increases the frequencies of
alleles that enhance survival and reproduction
• Adaptive evolution occurs as the match between
an organism and its environment increases
• Because the environment can change, adaptive
evolution is a continuous process
• Genetic drift and gene flow do not consistently
lead to adaptive evolution as they can increase
or decrease the match between an organism
and its environment
© 2011 Pearson Education, Inc.
Genetic Drift
• Genetic drift occurs when changes in gene
frequencies from one generation to another
occur because of chance events that occur
within a small population
– Example, seven heads and three tails in ten tosses is
no great feat, but 700 heads and 300 tails in 1000
tosses would earn you a trip to Vegas
• Genetic drift often occurs as a result of two
situations:
– bottleneck effect
– founder effect
• The bottleneck effect occurs when a larger
population is drastically reduced by a disaster.
– By chance, some alleles may be overrepresented
and others underrepresented among the survivors.
– Some alleles may be eliminated altogether.
– Genetic drift will
continue to impact
the gene pool until
the population is
large enough to
minimize the impact
of sampling errors.
• Bottlenecking is an important concept in
conservation biology of endangered species.
– This reduces individual variation and adaptability.
– For example, the genetic variation in the three
small surviving wild populations of cheetahs is very
low when compared to other mammals.
• Their genetic variation is
similar to highly inbred lab mice!
• The founder effect occurs when a new
population is started by only a few individuals.
– They do not represent the gene pool of the larger
population.
– Potentially, a population could be started by single
pregnant female or single seed.
• Genetic drift would continue from generation to
generation until the population grew large
enough for sampling errors to be minimal.
– Founder effects have been demonstrated in human
populations that started from a small group of
colonists.
• Gene flow is genetic exchange due to movement
of individuals or gametes between populations.
– If a wildflower population consisted entirely of white
flowers (yy alleles only) could be carried into a new
population that is all yellow, (Y alleles only) this
would increase the frequency of y alleles in the
second population in the next generation.
• Gene flow tends to reduce
differences between populations
(differences between population #1
and #2).
– If extensive enough, gene flow
can integrate neighboring
populations into a single
population with a common
genetic structure.
– The migration of people
throughout the world is
transferring alleles between
populations that were once
isolated, increasing gene flow.
Concept 23.4: Natural selection =
adaptive evolution
• Only natural selection consistently results
in adaptive evolution
• The phrases “struggle for existence” and
“survival of the fittest” are misleading as
they imply direct competition among
individuals
• Reproductive success is generally more
subtle and depends on many factors
Sexual Selection
• Sexual selection is
natural selection for
mating success
• It can result in sexual
dimorphism, marked
differences between
the sexes in
secondary sexual
characteristics
• Intrasexual selection is competition
among individuals of one sex (often
males) for mates of the opposite sex
• Intersexual selection, often called mate
choice, occurs when individuals of one sex
(usually females) are choosy in selecting
their mates
• What is the basis for female choice?
• The underlying bases of female choice is
probably not aesthetic.
– Recent research is investigating the
hypothesis that females use these sexual
advertisements to measure the general health
of a male (Good Genes Hypothesis).
– Individuals with infections or disease are likely
to have a relatively dull, disheveled plumage.
– For the female that chooses a healthy mate
the benefit is a greater probability of having
healthy offspring.