• A null model
• Describes a population that is not evolving
• If no other forces are at work, the frequencies of alleles and genotypes in a population's gene pool remain constant from generation to generation.

1. No selection (fitness)
2. Mating is random
3. Population is infinitely large
4. No gene flow from outside population
5. No mutation

• heterozygotes are more fit (violates "no selection" of null model)

Individuals with higher fitness contribute _____ gene copies to the next generation than less fit individuals.

Dependent on: reproduction or survival?

more;

reproduction

-Directional

-Disruptive

-Stabilizing

• the extreme phenotype is the most fit
• will change population mean
• will slightly decrease variation

• intermediate phenotypes are most fit
• will NOT change population mean
• will decrease variation
• most common form of selection

• two extreme phenotypes are more fit than intermediate phenotypes
• will NOT change population mean
• will increase variation

• It is a genetic variation that appears to confer no selective advantage or disadvantage
• Neutral variation cannot be removed by selection

• Maintains genetic variation in the form of hidden recessive alleles
• Recessive alleles in heterozygotes are not subject to selection

• Occurs when natural selection maintains stable frequencies or two or more phenotypic forms in a population
• Leads to a state called balance polymorphism
• Heterozygote advantage and frequency-dependent selection are two forms of balancing selection

• Causes mutations in hemoglobin but also confers malaria resistance
• Exemplifies the heterozygote advantage