Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels

문제 4

20장, 문제 4

Describe how natural selection can produce balanced polymorphism of allele frequencies through selection that favors heterozygotes.

검증된 단계별 안내

Understand the concept of balanced polymorphism, which occurs when two or more alleles are maintained in a population's gene pool at stable frequencies over time.

Recognize that heterozygote advantage (also called overdominance) is a form of natural selection where individuals with heterozygous genotypes have higher fitness than either homozygous genotype.

Set up the fitness values for the three genotypes: let the fitness of homozygote AA be

W_{AA}

, heterozygote Aa be

W_{Aa}

, and homozygote aa be

W_{aa}

, with

W_{Aa} > W_{AA}

and

W_{Aa} > W_{aa}

Use the concept that selection will increase the frequency of the heterozygote-favored allele combinations, but because both alleles contribute to heterozygotes, neither allele is lost, leading to a stable equilibrium of allele frequencies.

Explain that the equilibrium frequencies can be calculated by setting the change in allele frequency to zero and solving the equations derived from the relative fitness values, showing how natural selection maintains both alleles in the population.

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주요 개념

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Natural Selection

Natural selection is the process where individuals with traits better suited to their environment have higher survival and reproductive success. Over time, this leads to changes in allele frequencies within a population, favoring alleles that confer a fitness advantage.

Balanced Polymorphism

Balanced polymorphism occurs when two or more alleles are maintained in a population at stable frequencies due to selective advantages. This genetic diversity persists because heterozygotes have higher fitness than either homozygote, preventing any single allele from becoming fixed or lost.

Heterozygote Advantage

Heterozygote advantage is a form of selection where individuals with two different alleles (heterozygotes) have greater fitness than those with two identical alleles (homozygotes). This advantage maintains multiple alleles in the gene pool, as seen in cases like sickle cell trait providing malaria resistance.