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Chapter 16 Evolution of Populations
Summary
16 –1 Genes and Variation
Darwin’s theory of evolution by natural selection explained how
life on Earth changed, or evolved, over many generations. What
Darwin did not know was how heritable traits were passed down
through each generation. The study of genetics helps scientists
understand the relationship between inheritance and evolution.
Genetics supports Darwin’s ideas. Scientists know that genes
control traits and that many genes have at least two forms, or alleles. They also know that members of all species are heterozygous
for many genes. In genetic terms, evolution is any change in the
relative frequency of alleles in a population. A population is
a group of individuals of the same species that can interbreed.
Members of a population share a gene pool. A gene pool is all the
genes, and their alleles, in the population. The number of times the
allele occurs in a gene pool compared to the number of times that
other alleles for the same gene occur is the relative frequency of
the allele.
The two main sources of genetic variation are mutations and
gene shuffling.
• A mutation is any change in a sequence of DNA.
• Gene shuffling occurs during gamete formation. It can
produce millions of different gene combinations.
Both mutations and gene shuffling increase genetic variation by
increasing the number of different genotypes.
The number of phenotypes for a trait depends on how many
genes control the trait.
• A single-gene trait is a trait controlled by only one gene.
If there are two alleles for the gene, two or three genotypes
are possible. An example in humans of a single-gene trait is
the presence of a widow’s peak—a downward dip in the
center of the hairline. The allele for a widow’s peak is
dominant over the allele for a hairline with no peak.
As a result, there are only two phenotypes—having a
widow’s peak or not having one.
• A polygenic trait is controlled by two or more genes.
Each gene of a polygenic trait may have more than one
allele. Polygenic traits form many phenotypes. Variation
in a polygenic trait in a population often forms a bell-shaped
curve with most members near the middle. An example of
a polygenic trait is height in humans.
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16–2 Evolution as Genetic Change
Evolution of populations results from the effects of natural selection on individuals. Natural selection on single-gene traits can
lead to changes in allele frequencies and thus to evolution. The
process can cause an increase or a decrease in the relative frequency of an allele.
Natural selection on polygenic traits is more complex. Natural
selection on polygenic traits can occur in three ways:
• Directional selection occurs when individuals at one end of
the bell-shaped curve have higher fitness than individuals
near the middle or other end of the curve. Directional selection causes a shift in the curve toward the higher fitness end.
• Stabilizing selection occurs when individuals near the
middle of the curve have higher fitness than those at either
end. Stabilizing selection leads to a narrowing of the curve
near the middle.
• Disruptive selection occurs when individuals at the upper
and lower ends of the curve have higher fitness than those
near the middle. Disruptive selection forms a curve with a
peak at each end and a low point in the middle.
Natural selection is not the only source of evolutionary change.
In small populations, chance can cause alleles to become more or
less common. This kind of change in allele frequency is called
genetic drift. Genetic drift occurs when individuals with a
specific allele leave more descendants than other individuals,
just by chance. Over time, this can cause an allele to become
more or less common in a population. Genetic drift may also
occur when a small group of individuals moves into a new habitat.
By chance, the small group may have different relative allele
frequencies than did the original population. When this happens,
it is called the founder effect.
To understand how evolution occurs, scientists first asked,
“Under what conditions does evolution not occur?” The HardyWeinberg principle answers this question. The principle states
that allele frequencies in a population stay the same unless one or
more factors change the frequencies. Genetic equilibrium is the
condition in which allele frequencies remain constant. Five conditions are needed for a population to be in genetic equilibrium:
• random mating.
• large population size.
• no migrations.
• no mutations.
• no natural selection.
If all five conditions are met, relative allele frequencies will not
change. Evolution will not occur.
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16–3 The Process of Speciation
Speciation is the formation of new species. For one species to
evolve into two new species, the gene pools of two populations
must be separated. As new species evolve, populations become
reproductively isolated from one another. When members of
two populations can no longer interbreed and produce fertile
offspring, reproductive isolation has occurred. Reproductive
isolation takes three forms.
• Behavioral isolation occurs when populations have different
courtship or reproductive behaviors.
• Geographic isolation occurs when geographic barriers
separate populations. Such barriers can include mountains
or rivers.
• Temporal isolation occurs when populations reproduce
at different times.
Peter and Rosemary Grant proved that natural selection is still
causing finches on the Galápagos Islands to evolve. The Grants
showed that there was enough heritable variation in finch beaks
to provide raw material for natural selection. The couple also
showed that beak differences led to fitness differences. These
fitness differences have brought about directional selection.
Combining the Grants’ work and Darwin’s ideas, scientists
have come up with a hypothetical scenario for the evolution
of Galápagos finches. Speciation in the Galápagos finches
occurred by
• Founding of a new population A few finches may have
traveled from the mainland to one of the islands. There, they
survived and reproduced.
• Geographic isolation Some birds then moved to a second
island. The two populations were geographically isolated.
They no longer shared a gene pool.
• Changes in the new population’s gene pool Seed sizes on
the second island favored birds with larger beaks. So this
bird population evolved into a population with larger beaks.
• Reproductive isolation In time, the large-beaked birds
were reproductively isolated from birds on other islands and
evolved into a new species.
• Ecological competition If birds from the second island
cross back to the first, they live in competition. Individuals
that are most different from one another compete less and
are most able to reproduce. In time, this may lead to the
evolution of yet another species.
Evolution continues today. For example, some bacteria are
evolving resistance to certain drugs. Evolutionary theory can help
us understand these changes.
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