Selection,
in action.

1. Variation — individuals in a population differ from each other.
2. Heritability — some of that variation is heritable (passed via DNA).
3. Differential survival/reproduction — some variants survive and reproduce better than others in the current environment.
4. Differential offspring — favourable traits become more common in the next generation; unfavourable traits become rarer.

Over generations, the population evolves — its mean trait values shift, or it splits into new lineages. Without all four conditions, no evolution. With them, evolution is inevitable.

Biston betularia — peppered moths in 19th-century Britain. Two forms: light (camouflaged on lichen-covered trees) and dark/melanic (rare, conspicuous).

1. Industrial Revolution: pollution killed lichens; tree bark darkened with soot.
2. Light moths now stood out against dark bark — picked off by predators.
3. Dark moths became camouflaged — survived and reproduced more.
4. Frequency of dark moths rose dramatically; within decades dark moths dominated industrial areas.
5. When pollution was reduced, lichens returned, light moths recovered. Reversal of selection.

Two flavours:

• Intra-sexual selection — competition within a sex (usually males) for access to mates. Drives larger body size, weapons (antlers, tusks), territorial behaviour.
• Inter-sexual selection — choice between sexes (usually females choosing males). Drives elaborate ornaments — peacock tail, bird song, courtship displays.

Sexually selected traits are often costly to survival but spread anyway — because reproductive success matters more than longevity for fitness.

1. Random mutations occasionally produce resistance alleles in bacteria.
2. In the presence of antibiotic, resistant bacteria survive while susceptible ones die.
3. Resistant bacteria reproduce; resistance alleles become more common.
4. Eventually the antibiotic stops working — the bacterial population is dominated by resistant strains.

This is why overuse of antibiotics (in medicine and agriculture) accelerates resistance — and why developing new antibiotics is a perpetual scientific arms race.