Selection changes populations

Explanation

Mutation creates new alleles, while meiosis and sexual reproduction reshuffle alleles into new combinations. Individuals in a population can therefore differ in heritable traits before a new environmental pressure appears. The environment does not direct a population to produce whichever mutation it needs.

A selection pressure affects survival or reproductive success. For example, an antibiotic can reduce susceptible bacteria while resistant variants leave more descendants; a predator can favour camouflage that lowers detection. The advantage depends on the particular environment and may carry costs elsewhere.

If the difference is heritable, alleles associated with greater reproductive contribution can become more frequent in later generations. Survival without reproduction is not enough to increase an allele's transmission. Fitness in this context concerns reproductive success, not physical strength or moral value.

A population is the smallest unit that evolves because its inherited composition changes across generations. An individual can acclimatise or develop, but does not change the population's allele frequencies by deciding to adapt. Selection acts through differences among individuals, while evolution is recorded at the population level.

Name the type of selection from its effect on a trait. In directional selection, one extreme of the range is favoured, for example longer beaks when only large, hard seeds remain, so the mean of the trait shifts in one direction over generations. In stabilising selection, the intermediate form is favoured and the extremes are removed, so the mean stays the same and the variation narrows. In disruptive selection, both extremes are favoured over the intermediate, which can split a population into two forms.

Not every change in allele frequency is caused by selection. Genetic drift is a change in allele frequencies by chance, and it is strongest in small populations. In the founder effect, a few individuals start a new population, for example on an island, and carry only a small and unrepresentative sample of the source population's alleles. In a bottleneck, a population crashes to a few survivors, for example after a disaster, and the survivors' allele frequencies differ by chance from those before. Both reduce genetic diversity, and an allele can become common or be lost regardless of whether it is advantageous.