Mechanisms of Evolution

Core Concepts

Variation: the raw material

Evolution needs heritable variation, and two processes supply it. Mutation — changes in DNA sequence — is the ultimate source of new alleles; most mutations are neutral or harmful, a few are beneficial, and the rate is low but steady. Sexual reproduction shuffles existing variation: crossing over, independent assortment, and fertilization recombine alleles into new combinations every generation. (Asexual organisms get new variation only from mutation.) This is why sexual populations can respond to selection faster than asexual ones.

Natural selection: adaptation by differential reproduction

Natural selection acts on phenotypes — the traits you can observe — and changes genotype frequencies as a result. It is non-random: individuals with traits that improve survival and reproduction leave more offspring. Three classic modes (commonly taught):

• Directional selection favors one extreme — for example, peppered moths darkening during industrial pollution, or antibiotic resistance favoring bacteria that carry resistance alleles.
• Stabilizing selection favors the middle and trims the extremes — for example, human birth weight: very small and very large babies face higher risk.
• Diversifying (disruptive) selection favors both extremes — for example, birds with very small or very large beaks when intermediate sizes are at a disadvantage — and can eventually contribute to speciation.

Genetic drift: change by chance

Genetic drift is random change in allele frequencies, strongest in small populations. It does not adapt populations; it simply removes or fixes alleles by sampling luck. Two famous cases:

• The bottleneck effect: a disaster sharply reduces population size; the survivors carry only a fraction of the original diversity, and the population that recovers is less diverse (cheetahs and northern elephant seals are commonly taught examples).
• The founder effect: a small group colonizes a new area; its gene pool reflects the founders, not the source population (certain genetic disorders common in isolated communities are commonly taught examples).

Gene flow: migration mixes gene pools

Gene flow (migration) moves alleles between populations when individuals move and breed. It tends to make populations more similar, and can introduce new alleles or dilute locally adapted ones. Gene flow can rescue a small, inbred population by adding diversity, or swamp a locally adapted one by diluting its specializations.

Non-random mating and sexual selection

If individuals choose mates by phenotype, allele frequencies change in ways selection alone would not predict. Sexual selection is a special case in which the trait itself affects mating success: peacock tails, deer antlers, bird song. It can produce traits that seem wasteful — even dangerous — because they advertise quality to mates.

Hardy-Weinberg equilibrium: the null model

The Hardy-Weinberg equilibrium describes a population in which allele frequencies do not change. The conditions are: no mutation, random mating, no natural selection, an infinitely large population (so no drift), and no gene flow. Under those conditions, allele frequencies remain constant and genotype frequencies are given by the equations:

p + q = 1 (allele frequencies)

p² + 2pq + q² = 1 (genotype frequencies)

where p and q are the frequencies of the two alleles of a gene. Real populations always violate at least one condition, so Hardy-Weinberg is a baseline: if measured genotype frequencies do not match p² + 2pq + q², something — selection, drift, migration, or non-random mating — is acting, meaning evolution is occurring.

A classic calculation: if the frequency of a recessive disorder is 1 in 2,500 (q² = 0.0004), then q = 0.02, p = 0.98, and the carrier frequency 2pq ≈ 0.039 — about 1 in 25 people (commonly taught example; values are illustrative, not a claim about any specific population).

Common Confusions