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Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 20 - Population Genetics and Evolution at the Population, Species, and Molecular LevelsProblem 4
Chapter 20, Problem 4

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

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1
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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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.
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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.
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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.
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Related Practice
Textbook Question

Identify and describe the evolutionary forces that can cause allele frequencies to change from one generation to the next.

Textbook Question

What insights have analyses of human mitochondrial DNA provided into our recent evolutionary past?

Textbook Question

What lines of evidence support the hypothesis that modern humans evolved in Africa and then subsequently migrated throughout the globe?

Textbook Question

Discuss how both gains and losses of regulatory elements may lead to human-specific traits.

Textbook Question

Thinking creatively about evolutionary mechanisms, identify at least two schemes that could generate allelic polymorphism in a population. Do not include the processes described in the answer to Problem 4.

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Textbook Question

How do copy-number variants arise? Do they account for more polymorphism than SNPs within the human population?