Chapter 16: The Evolution of Populations and Speciation

Objectives:• Describe two causes of genotypic variation in a population• Explain how to compute allele and phenotype frequency• Explain Hardy-Weinberg genetic equilibrium• List five conditions that can cause evolution to take place• What is genetic drift and how does it affect endangered species• Identify three types of reproductive isolation

Single-Gene and Polygenic Trait• A bell-shaped curve is typical of polygenic

traits.• A bell-shaped curve is also called normal

distribution.

•Sources of Genetic Variation–The two main sources of genetic variation are mutations and recombination that results from sexual reproduction.

Sources of Genetic Variation•Mutations– A mutation is any change in a sequence of

DNA.– Mutations occur because of mistakes in DNA

replication or as a result of radiation or chemicals in the environment.

– Mutations do not always affect an organism’s phenotype.

Sources of Genetic Variation•Recombination– Most heritable differences are due to gene

shuffling.– Crossing-over increases the number of

genotypes that can appear in offspring.– Sexual reproduction produces different

phenotypes, but it does not change the relative frequency of alleles in a population.

•How Common Is Genetic Variation?• Many genes have at least two forms, or alleles.• All organisms have genetic variation that is “invisible”

because it involves small differences in biochemical processes.

• An individual organism is heterozygous for many genes.

•Variation and Gene Pools• Genetic variation is studied in populations. • A population is a group of individuals of the same

species that interbreed. • A gene pool consists of all genes, including all the

different alleles, that are present in a population.

Variation and Gene Pools

•The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. •Relative frequency is often expressed as a percentage. •Phenotype frequency is that trait divided by the number of individuals in a population.•Allele Frequency is that allele divided by the number of alleles in a population

Variation and Gene Pools

•Gene Pool for Fur Color in MiceSample Population Frequency of Alleles

allele forbrown fur

allele forblack fur

Evolution Versus Genetic Equilibrium

• The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

• When allele frequencies remain constant it is called genetic equilibrium.

Evolution Versus GeneticEquilibrium

•What five conditions are needed to maintain genetic equilibrium?

Evolution Versus Genetic Equilibrium

Five conditions are required to maintain genetic equilibrium from generation to generation:1. there must be random mating,

2. the population must be very large,

3. there can be no movement into or out of the population,

4. there can be no mutations, and

5. there can be no natural selection.

Disruption of Genetic Equilibrium

•No Mutations– If genes mutate, new alleles may be

introduced into the population, and allele frequencies will change.

Disruption of Genetic Equilibrium

•No net Migration – Because individuals may bring new alleles

into a population, there must be no movement of individuals into or out of a population.

– The population's gene pool must be kept together and kept separate from the gene pools of other populations.

•Large Population/ Genetic Drift– Genetic drift is the phenomenon by which allele

frequencies in a population changes as a result of random events, or chance.

– Genetic drift has less effect on large populations than on small ones.

– Allele frequencies of large populations are less likely to be changed through the process of genetic drift.

Disruption of Genetic Equilibrium

•Non-Random Mating– Random mating ensures that each individual

has an equal chance of passing on its alleles to offspring.

– In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits.

Disruption of Genetic Equilibrium

•Natural Selection– All genotypes in the population must have

equal probabilities of survival and reproduction.

– No phenotype can have a selective advantage over another.

– There can be no natural selection operating on the population.

Disruption of Genetic Equilibrium

Natural Selection onPolygenic Traits

•Natural selection can affect the distributions of phenotypes in any of three ways:– directional selection– stabilizing selection– disruptive selection

Natural Selection onPolygenic Traits

•Directional Selection – When individuals at one end of the curve

have higher fitness than individuals in the middle or at the other end, directional selection takes place.

– The range of phenotypes shifts as some individuals survive and reproduce while others do not.

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Natural Selection onPolygenic Traits

•In this case, birds with larger beaks have higher fitness. Therefore, the average beak size increases.

Natural Selection onPolygenic Traits

–Stabilizing Selection • When individuals near the center of the curve have

higher fitness than individuals at either end of the curve, stabilizing selection takes place.

• This keeps the center of the curve at its current position, but it narrows the overall graph.

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Natural Selection on Polygenic Traits•Human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average.

Natural Selection on Polygenic Traits•Disruptive Selection – When individuals at the upper and lower ends of

the curve have higher fitness than individuals near the middle, disruptive selection takes place.

– If the pressure of natural selection is strong enough and long enough, the curve will split, creating two distinct phenotypes.

Natural Selection on Polygenic Traits•If average-sized seeds become scarce, a bird population will split into two groups: one that eats small seeds and one that eats large seeds.

Sexual Selection

• When selecting a mate females look for favorable traits.

• The genes of successful reproducers, rather than those of merely successful survivors, are amplified through natural selection.

Isolating mechanisms

• Geographical Isolation– Geographic isolation occurs when two

populations are separated by geographic barriers such as rivers or mountains.

• Reproductive Isolation– When the members of two populations cannot

interbreed and produce fertile offspring, reproductive isolation has occurred.

Assignment

• P.314 • Questions 1-5, 10,12,15,21, and 25.