natural selection
Imagine a meadow where slightly faster rabbits more often escape foxes and so leave more offspring. Over generations, the genes for speed become more common simply because their carriers reproduced more. That non-random, outcome-driven sorting is natural selection: differential survival and reproduction tied to inherited traits.
In genetic terms, natural selection is a change in allele frequencies caused by consistent differences in fitness among genotypes. Alleles that help their carriers survive and reproduce in a given environment rise in frequency; those that hinder reproduction decline. Crucially, the trait must be heritable for selection to leave a lasting genetic mark.
Selection comes in flavors. Directional selection pushes a trait one way; stabilizing selection favors the middle and trims extremes; disruptive selection favors both extremes over the average. Each leaves a characteristic fingerprint on the distribution of traits and alleles.
Unlike drift, selection has direction, but it has no goal or foresight—it can only act on the variation already present and only with respect to the current environment. An allele favored today can become a liability when conditions change.
Selection acts on phenotypes but is recorded in allele frequencies. It needs three ingredients: variation, heritability, and a difference in reproductive success linked to that variation.