Evolution of Populations
How Common is Genetic 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.
Variation and Gene Pools
•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 that 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.
Relative Frequencies of Alleles
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
Sources of Genetic Variation
•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 (genetic makeup
of an organism).
Single-Gene and Polygenic Traits
•The number of phenotypes (physical characteristics of an organisms) for
a trait depends on how many genes control the trait.
•A single-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.
Phenotypes for Single-Gene Trait
Frequency of Phenotype
(%)
100
80
60
40
20
0
Widow’s peak
Phenotype
No widow’s peak
Single-Gene and Polygenic Traits
•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 bellshaped curve with most members near the middle.
– An example of a polygenic trait is height in humans
Frequency of Phenotype
Generic Bell Curve for Polygenic Trait
Phenotype (height)
Natural Selection on Single-Gene Traits
•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 decrease in the relative
frequency of an allele.
Natural Selection on Polygenic Traits
•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 the other end of the curve.
• Directional selection causes a shift in the curve toward
the higher fitness end.
Graph of Directional Selection
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Natural Selection on Polygenic Traits
– 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.
Graph of Stabilizing Selection
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Natural Selection on Polygenic Traits
– 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.
Graph of Disruptive Selection
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genetic Drift
•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
•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.
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Evolution Versus Genetic Equilibrium
•To understand how evolution occurs, scientists first asked,
“Under what conditions does evolution not occur?”
•The Hardy-Weinberg principle answers this questions.
•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.
Evolution Versus Genetic Equilibrium
•Five conditions are needed for a population to be in genetic
equilibrium.
•
1. random mating
•
2. large population size
•
3. no migration
•
4. no mutations
•
5. no natural selection
•If all five conditions are met, relative allele frequencies will not
change. Evolution will not occur.
Speciation
•Speciation is the formation of new species.
•For one species to evolve into two new species, the gene pools
of two population must be separated.
Isolating Mechanisms
•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.
Concept Map
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
Testing Natural Selection in Nature
•Peter and Rosemary Grant proved that natural selection is still
causing finches on the Galapagos 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.
Galapagos Island Finches
Speciation in Darwin’s Finches
•Combining the Grant’s and Darwin’s ideas, scientists have come up with a
hypothetical scenario for the evolution of Galapagos finches.
•Speciation in the Galapagos 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.
Speciation in Darwin’s Finches
– 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.
Studying Evolution Since Darwin
•Evolution continues today.
•For example, some bacteria are evolving resistance to certain
drugs.
•Evolutionary theory can help us understand these changes.