Practice question

Balancing selection maintains multiple alleles stably in population rather than favoring single variant toward fixation and loss. Heterozygote advantage, also called overdominance, is a classic form of balancing selection where heterozygous genotype fitness exceeds both homozygous genotypes, preventing elimination of either allele. This opposes directional and disruptive selection that reduce variation over time. Classic examples include sickle cell polymorphism and MHC diversity maintained by pathogen pressure. By selecting against both homozygote classes, heterozygote superiority preserves stable polymorphism, categorizing it under balancing selection mechanisms counteracting genetic drift.