Adaptive Evolution

Core Concepts

How selection produces adaptation

Adaptive evolution by natural selection needs three ingredients: heritable variation in a trait, differential survival or reproduction linked to that variation, and time. The most successful reproducers — the most fit, where fitness means reproductive contribution to the next generation — pass on their alleles disproportionately: fitness is reproductive success, not strength, speed, or longevity.

The three modes of selection on quantitative traits

Picture a bell-shaped distribution of a trait (like beak depth, height, or birth weight):

• Directional selection favors one extreme, shifting the mean toward it while retaining the distribution's shape. Example: in a drought, finches with deeper beaks crack tougher remaining seeds, and average beak depth rises (the commonly taught Galápagos finch studies).
• Stabilizing selection favors the intermediate and selects against both extremes. The mean holds, but the curve narrows — variance decreases. Human birth weight is the standard example: very low and very high birth weights carry higher risk (commonly taught).
• Diversifying (disruptive) selection favors both extremes against intermediates, and the distribution can split into two peaks — a first step toward two distinct forms. In some seedcracker populations, very small and very large beaks each handle different seeds, while intermediate beaks are inefficient (commonly taught).

Exam rule: ask where fitness is highest — one extreme, the middle, or both ends.

Sexual selection: selection on mating success

Sexual selection is natural selection acting on the ability to obtain mates. It comes in two forms:

• Intrasexual selection — competition within one sex (usually males) for access to mates; winners reproduce, losers may not. This drives weapons: the antlers of male deer, the horns of male beetles.
• Intersexual selection — mate choice, typically females choosing among males. This drives ornaments: the peacock's tail, the songs of male birds.

The good genes hypothesis proposes that females prefer exaggerated ornaments because a male that grows a huge tail or performs a complex display despite parasites and poor nutrition is advertising genetic quality — offspring inherit "good genes," so preference and ornament co-evolve. Sexual selection can even produce traits that reduce survival (a giant tail slows escape): the trait persists because it boosts mating success enough to outweigh the survival cost.

Balancing selection: keeping variation

Most selection removes variation, but balancing selection actively maintains it:

• Heterozygote advantage: heterozygotes (Aa) are fitter than either homozygote. The classic case is the sickle-cell trait: in malarial regions, heterozygotes have a survival advantage against malaria while the homozygous states carry serious costs (commonly taught — an advantage of the heterozygous genotype in a specific environment, not a "benefit" of the disorder). Both alleles persist, so the population stays polymorphic.
• Frequency-dependent selection: fitness depends on how common a phenotype is; rare types gain an advantage as they get rarer. In some scale-eating cichlids, individuals attack from the left or the right; the rarer "handedness" succeeds because prey habituate to the common direction, keeping both morphs balanced (commonly taught).

No perfect organism

Adaptive evolution has hard limits: selection works only with existing variation (it cannot invent alleles), traits involve trade-offs (a heavier shell protects but costs energy), the environment changes (today's adaptation can become tomorrow's liability), and drift and gene flow can override selection. Evolution has no goal — it is a population's best current fit to a moving target, which is why "perfect" organisms do not exist.

Common Confusions