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.

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

Relative Frequencies of Alleles

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.

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Widow’s peak No widow’s peak

Phenotype

Phenotypes for Single-Gene Trait

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 bell-

shaped curve with most members near the middle.– An example of a polygenic trait is height in humans

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Phenotype (height)

Generic Bell Curve for Polygenic Trait

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.

Directional Selection

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

Graph of Directional Selection

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.

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Birth Weight

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extremes keep curve narrow and in same

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Graph of Stabilizing Selection

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

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.

Disruptive Selection

Largest and smallest seeds become more common.

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Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Graph of Disruptive Selection

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Low mortality, high fitness

High mortality, low fitness

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.

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

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

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Reproductive Isolation

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew 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.