Natural selection

Natural selection is the process in which individuals with inherited traits better suited to their environment tend to survive and reproduce more than others, so those traits become more common in later generations. It operates whenever three conditions are met: individuals in a population vary in some trait, the variation is at least partly heritable, and more offspring are produced than the environment can support, so not all of them survive and reproduce equally.

Selection acts on individuals, but its results show up in populations as changes in allele frequencies over time. An organism's success is measured as its fitness, its contribution of offspring to the next generation relative to others. Selection can favor the average form and remove extremes (stabilizing selection), push a population toward one extreme (directional selection), or favor both extremes over the middle (disruptive selection). It is the only known mechanism that produces adaptations, but it can only work with variation that already exists, supplied by mutation and sexual reproduction.

Classic examples include shifts in beak size among Galápagos finches after droughts changed their seed supply, the rise of dark peppered moths in soot-blackened industrial England, and the spread of antibiotic resistance, where a drug favors resistant bacteria that were already present.