11.2 Selection, Gene Pools and Hardy–Weinberg

Natural selection requires variation, heritability and unequal reproductive success. Mutation creates new alleles. Crossing over and independent assortment during meiosis recombine alleles, and random fertilization joins two independently produced gametes. Environmental effects also create phenotypic variation, but only a genetic component can be transmitted in the relevant way. Selection acts on phenotypes while evolution is measured as change in allele frequencies.

Organisms generally produce more offspring than available resources can support. Individuals compete directly or indirectly for food, light, water, territory and mates, while predators, parasites, disease and physical conditions impose additional selection pressures. A phenotype that performs better under those conditions tends, on average, to leave more fertile offspring. Its associated alleles therefore contribute a larger fraction of the next generation.

Fitness is relative and environmental. A dark insect may be camouflaged on soot-darkened bark but exposed on pale bark. An antibiotic-resistance allele may be advantageous during treatment yet impose a metabolic cost without the drug. Natural selection does not anticipate future conditions, create needed mutations or aim toward perfection. Existing heritable variants differ in reproductive success under current conditions.

Resistance illustrates the complete mechanism. Mutation or horizontal gene transfer produces variation before or during exposure. A pesticide or antibiotic kills susceptible organisms more readily; resistant organisms survive and reproduce, transmitting resistance alleles or plasmids. Repeated exposure increases the resistant fraction. The chemical did not teach individual organisms to resist it: it changed which variants supplied descendants.

Sexual selection is differential mating success. Intrasexual selection occurs through competition within one sex, while intersexual selection involves mate choice. Antlers, calls, displays or coloration may increase access to mates even when they reduce survival. The apparent conflict disappears when fitness is considered as total genetic contribution to offspring: a costly display can spread if its mating advantage exceeds its survival cost.