Chapter 23 The Evolution of Populations
Overview: The Smallest Unit of Evolution
• One common misconception about evolution is that organisms
evolve, in a Darwinian sense, during their lifetimes.
• Natural selection does act on individuals. Each individual’s
combination of inherited traits affects its survival and its reproductive
success relative to other individuals in the population.
• However, the evolutionary impact of natural selection is only apparent
in the changes in a population of organisms over time.
• It is the population, not the individual, that evolves.
• Consider the example of bent grass (Agrostis tenuis) growing on the
tailings of an abandoned mine. These tailings are rich in toxic heavy
• While many bent grass seeds land on the mine tailings each year, the
only plants that germinate, grow, and reproduce are those that
possess genes enabling them to tolerate metallic soils.
• These plants tend to produce metal-tolerant offspring.
• Individual plants do not evolve to become more metal-tolerant during
Concept 23.1 Population genetics provides a foundation for studying
• Darwin proposed a mechanism for change in species over time.
• What was missing from Darwin’s explanation was an understanding
of inheritance that could explain how chance variations arise in a
population while also accounting for the precise transmission of these
variations from parents to offspring.
• The widely accepted hypothesis of the time—that the traits of parents
are blended in their offspring—would eliminate the differences in
individuals over time. • Just a few years after Darwin published On the Origin of Species,
Gregor Mendel proposed a model of inheritance that supported
• Mendel’s particulate hypothesis of inheritance stated that parents
pass on discrete heritable units (genes) that retain their identities in
• Although Gregor Mendel and Charles Darwin were contemporaries,
Darwin never saw Mendel’s paper, and its implications were not
understood by the few scientists who did read it at the time.
• Mendel’s contribution to evolutionary theory was not appreciated until
half a century later.
The modern evolutionary synthesis integrated Darwinian selection
and Mendelian inheritance.
• When Mendel’s research was rediscovered in the early 20th century,
many geneticists believed that his laws of inheritance conflicted with
Darwin’s theory of natural selection.
• Darwin emphasized quantitative characters, those that vary along a
• These characters are influenced by multiple loci.
• Mendel and later geneticists investigated discrete “either-or” traits.
• It was not obvious that there was a genetic basis to quantitative
• Within a few decades, geneticists determined that quantitative
characters are influenced by multiple genetic loci and that the alleles
at each locus follow Mendelian laws of inheritance.
• These discoveries helped reconcile Darwin’s and Mendel’s ideas and
led to the birth of population genetics, the study of how populations
change genetically over time.
• A comprehensive theory of evolution, the modern synthesis, took
form in the early 1940s. • It integrated discoveries and ideas from paleontology, taxonomy,
biogeography, and population genetics.
• The first architects of the modern synthesis included statistician R. A.
Fisher, who demonstrated the rules by which Mendelian characters
are inherited, and biologist J. B. S. Haldane, who explored the rules
of natural selection. Later contributors included geneticists
Theodosius Dobzhansky and Sewall Wright, biogeographer and
taxonomist Ernst Mayr, paleontologist George Gaylord Simpson, and
botanist G. Ledyard Stebbins.
• The modern synthesis emphasizes:
• The importance of populations as the units of evolution.
• The central role of natural selection as the most important
mechanism of adaptive evolution.
• The idea of gradualism to explain how large changes can evolve as
an accumulation of small changes over long periods of time.
• While many evolutionary biologists are now challenging some of the
assumptions of the modern synthesis, it has shaped our ideas about
how populations evolve.
A population’s gene pool is defined by its allele frequencies.
• A population is a localized group of individuals that belong to the
• One definition of a species is a group of natural populations whose
individuals have the potential to interbreed and produce fertile
• Populations of a species may be isolated from each other and rarely
exchange genetic material.
• Members of a population are far more likely to breed with members of
the same population than with members of other populations.
• Individuals near the population’s center are, on average, more closely
related to one another than to members of other populations. • The total aggregate of genes in a population at any one time is called
the population’s gene pool.
• It consists of all alleles at all gene loci in all individuals of a
• If only one allele exists at a particular locus in a population, that allele
is said to be fixed in the gene pool, and all individuals will be
homozygous for that gene.
• If there are two or more alleles for a particular locus, then individuals
can be either homozygous or heterozygous for that gene.
• Each allele has a frequency in the population’s gene pool.
• For example, imagine a population of 500 wildflower plants with two
alleles (CR and CW) at a locus that codes for flower pigment.
• Suppose that in the imaginary population of 500 plants, 20 (4%) are
homozygous for the CW allele (CWCW) and have white flowers.
• Of the remaining plants, 320 (64%) are homozygous for the CR allele
(CRCR) and have red flowers.
• These alleles show incomplete dominance. 160 (32%) of the plants
are heterozygous (CRCW) and produce pink flowers.
• Because these plants are diploid, the population of 500 plants has
1,000 copies of the gene for flower color.
• The dominant allele (CR) accounts for 800 copies (320 × 2 for CRCR
+ 160 × 1 for CRCW).
• The frequency of the CR allele in the gene pool of this population is
800/1,000 = 0.8, or 80%.
• The CW allele must have a frequency of 1.0 ? 0.8 = 0.2, or 20%.
• When there are two alleles at a locus, the convention is to use p to
represent the frequency of one allele and q to represent the
frequency of the other.
• Thus p, the frequency of the CR allele in this population, is 0.8.
• The frequency of the CW allele, represented by q, is 0.2. The Hardy-Weinberg Theorem describes a nonevolving population.
• The Hardy-Weinberg theorem describes the gene pool of a
• This theorem states that the frequencies of alleles and genotypes in a
population’s gene pool will remain constant over generations unless
acted upon by agents other than Mendelian segregation and
recombination of alleles.
• The shuffling of alleles by meiosis and random fertilization has no
effect on the overall gene pool of a population.
• In our imaginary wildflower population of 500 plants, 80% (0.8) of the
flower color alleles are CR, and 20% (0.2) are CW.
• How will meiosis and sexual reproduction affect the frequencies of
the two alleles in the next generation?
• We assume that fertilization is completely random and all male-
female mating combinations are equally likely.
• Because each gamete has only one allele for flower color, we expect
that a gamete drawn from the gene pool at random has a 0.8 chance
of bearing an CR allele and a 0.2 chance of bearing an CW allele.
• Suppose that the individuals in a population not only donate gametes
to the next generation at random, but also mate at random. In other
words, all male-female matings are equally likely.
• The allele frequencies in this population will not change from one
generation to the next. Its genotype frequencies, which can be
predicted from the allele frequencies, will also remain unchanged.
• For the flower-color locus, the population’s genetic structure is in a
state of Hardy-Weinberg equilibrium.
• Using the rule of multiplication, we can determine the frequencies of
the three possible genotypes in the next generation.
• The probability of picking two CR alleles (to obtain a CRCR
genotype) is 0.8 × 0.8 = 0.64, or 64%. • The probability of picking two CW alleles (to obtain a CWCW
genotype) is 0.2 × 0.2 = 0.04, or 4%.
• Heterozygous individuals are either CRCW or CWCR, depending on
whether the CR allele arrived via sperm or egg.
• The probability of being heterozygous (with a CRCW genotype) is 0.8
× 0.2 = 0.16 for CRCW, 0.2 × 0.8 = 0.16 for CWCR, and 0.16 + 0.16
= 0.32, or 32%, for CRCW + CWCR.
• As you can see, the processes of meiosis and random fertilization
have maintained the same allele and genotype frequencies that
existed in the previous generation.
• The Hardy-Weinberg theorem states that the repeated shuffling of a
population’s gene pool over generations does not increase the
frequency of one allele over another.
• Theoretically, the allele frequencies in our flower population should
remain at 0.8 for CR and 0.2 for CW forever.
• To generalize the example, in a population with two alleles with
frequencies of p and q, the combined frequencies must add to 100%.
• Therefore p + q = 1.
• If p + q = 1, then p = 1 ? q and q = 1 ? p.
• In the wildflower example, p is the frequency of red alleles (CR) and q
is the frequency of white alleles (CW).
• The probability of generating an CRCR offspring is p2 (an application
of the rule of multiplication).
• In our example, p = 0.8 and p2 = 0.64.
• The probability of generating a CWCW offspring is q2.
• In our example, q = 0.2 and q2 = 0.04.
• The probability of generating a CRCW offspring is 2pq.
• In our example, 2 × 0.8 × 0.2 = 0.32.
• The genotype frequencies must add up to 1.0: p2 + 2pq + q2 = 1.0
• For the wildflowers, 0.64 + 0.32 + 0.04 = 1.0.
• This general formula is the Hardy-Weinberg equation.
• Using this formula, we can calculate frequencies of alleles in a gene
pool if we know the frequency of genotypes, or the frequency of
genotypes if we know the frequencies of alleles.
Five conditions must be met for a population to remain in Hardy-
• The Hardy-Weinberg theorem describes a hypothetic population that
is not evolving. However, real populations do evolve, and their allele
and genotype frequencies do change over time.
• That is because the five conditions for nonevolving populations are
rarely met for long in nature.
• A population must satisfy five conditions if it is to remain in Hardy-
1. Extremely large population size. In small populations, chance
fluctuations in the gene pool can cause genotype frequencies to
change over time. These random changes are called genetic
2. No gene flow. Gene flow, the transfer of alleles due to the
migration of individuals or gametes between populations, can
change the proportions of alleles.
3. No mutations. Introduction, loss, or modification of genes will
alter the gene pool.
4. Random mating. If individuals pick mates with certain
genotypes, or if inbreeding is common, the mixing of gametes
will not be random.
5. No natural selection. Differential survival or reproductive
success among genotypes will alter their frequencies.
• Evolution usually results when any of these five conditions are not
met. • Although natural populations are rarely, if ever, in true Hardy-
Weinberg equilibrium, the rate of evolutionary change in many
populations is so slow that they appear to be close to equilibrium.
• In such cases, we can use the Hardy-Weinberg equation to estimate
genotype and allele frequencies.
• We can use the theorem to estimate the percentage of the human
population that carries the allele for the inherited disease
• About 1 in 10,000 babies born in the United States is born with PKU,
a metabolic condition that results in mental retardation and other
problems if left untreated.
• The disease is caused by a recessive allele.
• Is the U.S. population in Hardy-Weinberg equilibrium with respect to
the PKU gene?
1. The U.S. population is very large.
2. Populations outside the United States have PKU allele
frequencies similar to those seen in the United States, so gene
flow will not alter allele frequencies significantly.
3. The mutation rate for the PKU gene is very low.
4. People do not choose their partners based on whether or not
they carry the PKU allele, and inbreeding (marriage to close
relatives) is rare in the United States.
5. Selection against PKU only acts against the rare heterozygous
• From the epidemiological data, we know that frequency of
homozygous recessive individuals (q2 in the Hardy-Weinberg
theorem) = 1 in 10,000, or 0.0001.
• The frequency of the recessive allele (q) is the square root of 0.0001
• The frequency of the dominant allele (p) is p = 1 ? q, or 1 ? 0.01 =
0.99. • The frequency of carriers (heterozygous individuals) is 2pq = 2 × 0.99
× 0.01 = 0.0198, or about 2%.
• Thus, about 2% of the U.S. population carries the PKU allele.
Concept 23.2 Mutation and sexual recombination produce the
variation that makes evolution possible
New genes and new alleles originate only by mutation.
• A mutation is a change in the nucleotide sequence of an organism’s
• Most mutations occur in somatic cells and are lost when the individual
• Only mutations in cell lines that form gametes can be passed on to
offspring, and only a small fraction of these spread through
populations and become fixed.
• A new mutation that is transmitted in a gamete to an offspring can
immediately change the gene pool of a population by introducing a
• A point mutation is a change of a single base in a gene.
• Point mutations can have a significant impact on phenotype, as in the
case of sickle-cell disease.
• However, most point mutations are harmless.
• Much of the DNA in eukaryotic genomes does not code for protein
• However, some noncoding regions of DNA do regulate gene
• Changes in these regulatory regions of DNA can have profound
• Because the genetic code is redundant, some point mutations in
genes that code for proteins may not alter the protein’s amino acid
composition. • On rare occasions, a mutant allele may actually make its bearer
better suited to the environment, increasing reproductive success.
• This is more likely when the environment is changing.
• Some mutations alter gene number or sequence.
• Chromosomal mutations that delete or rearrange many gene loci at
once are almost always harmful.
• In rare cases, chromosomal rearrangements may be beneficial.
• For example, the translocation of part of one chromosome to a
different chromosome could link genes that act together to positive
• Gene duplication is an important source of new genetic variation.
• Small pieces of DNA can be introduced into the genome through the
activity of transposons.
• Such duplicated segments can persist over generations and provide
new loci that may eventually take on new functions by mutation and
• New genes may also arise when the coding subsections of genes
known as exons are shuffled within the genome, within a single locus
or between loci.
• Such beneficial increases in gene number appear to have played a
major role in evolution.
• For example, mammalian ancestors carried a single gene for
detecting odors that has been duplicated though various mutational
• Modern humans have close to 1,000 olfactory receptor genes.
• 60% of these genes have been inactivated in humans, due to
• Mice, who rely more on their sense of smell, have lost only 20% of
their olfactory receptor genes.
• Mutation rates vary from organism to organism. • Mutation rates are low in animals and plants, averaging about 1
mutation in every 100,000 genes per generation.
• In microorganisms and viruses with short generation spans, mutation
rates are much higher and can rapidly generate genetic variation.
Sexual recombination also produces genetic variation.
• On a generation-to-generation timescale, sexual recombination is far
more important than mutation in producing the genetic differences
that make adaptation possible.
• Sexual reproduction rearranges alleles into novel combinations every
• Bacteria and viruses can also undergo recombination, but they do so
less regularly than animals and plants.
• Bacterial and viral recombination may cross species barriers.
Concept 23.3 Natural selection, genetic drift, and gene flow can alter a
population’s genetic composition
• Although new mutations can modify allele frequencies, the change
from generation to generation is very small.
• Recombination reshuffles alleles but does not change their
• Three major factors alter allele frequencies to bring about
evolutionary change: natural selection, genetic drift, and gene flow.
Natural selection is based on differential reproductive success.
• Individuals in a population vary in their heritable traits.
• Those with variations better suited to the environment tend to
produce more offspring than those with variations that are less well
• As a result of selection, alleles are passed on to the next generation
in frequencies different from their relative frequencies in the present
population. • Imagine that in our imaginary wildflower population, white flowers are
more visible to herbivorous insects and thus have lower survival.
Imagine that red flowers are more visible to pollinators.
• Such differences in survival and reproductive success would disturb
the Hardy-Weinberg equilibrium. The frequency of the CW allele
would decline and the frequency of the CR allele would increase.
Genetic drift results from chance fluctuations in allele frequencies in
• Genetic drift occurs when changes in gene frequencies from one
generation to another occur because of chance events (sampling
errors) that occur in small populations.
• For example, you would not be too surprised if a thrown coin
produced seven heads and three tails in ten tosses, but you would be
surprised if you saw 700 heads and 300 tails in 1,000 tosses—you
would expect close to 500 of each.
• The smaller the sample, the greater the chance of deviation from the
• In a large population, allele frequencies will not change from
generation to generation by chance alone.
• However, in a small wildflower population with a stable size of only
ten plants, genetic drift can completely eliminate some alleles.
• Genetic drift at small population sizes may occur as a result of two
situations: the bottleneck effect or the founder effect.
• The bottleneck effect occurs when the numbers of individuals in a
large population are 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 change the gene pool until the population
is large enough to eliminate the effect of chance fluctuations. • The bottleneck effect is an important concept in conservation biology
of endangered species.
• Populations that have suffered bottleneck incidents have lost genetic
variation from the gene pool.
• This reduces individual variation and may reduce adaptation.
• For example, in the 1890s, hunters reduced the population of
northern elephant seals in California to 20 individuals.
• Now that it is a protected species, the population has increased to
more than 30,000.
• However, a study of 24 gene loci in a representative sample of seals
showed no variation. One allele had been fixed for each gene.
• Populations of the closely related southern elephant seal, which did
not go through a bottleneck, show abundant genetic variation.
• The founder effect occurs when a new population is started by only a
few individuals who do not represent the gene pool of the larger
• At an extreme, a population could be started by a single pregnant
female or single seed with only a tiny fraction of the genetic variation
of the source population.
• Genetic drift would conti