Populations Exam 6 Practice

Balancing selection keeps multiple alleles in a population by different mechanisms. In overdominance, the heterozygote genotype has higher fitness than either homozygote, so both alleles are maintained at stable frequencies because offspring with the heterozygous combination are favored. This directly preserves genetic variation since neither allele is lost. In frequency-dependent selection, an allele’s fitness depends on how common it is in the population. When an allele is rare, it has a fitness advantage; as it becomes more frequent, its advantage diminishes. This negative frequency dependence prevents any one allele from sweeping to fixation and can maintain a polymorphism or even cause cycles in allele frequencies over time. So, overdominance maintains both alleles via heterozygote advantage, while frequency-dependent selection maintains alleles because their fitness changes with how common or rare they are.