Natural Selection

Conditions for natural selection (Darwin-Wallace):

1. Variation — individuals in a population differ in characteristics.
2. Overproduction of offspring — more young are produced than can possibly survive.
3. Struggle for existence — limited resources cause competition.
4. Differential survival and reproduction — individuals best suited to current conditions survive and reproduce more.
5. Heritability — adaptive traits are passed to offspring; over generations, the population shifts.

Mathematically: if a heritable trait gives a small reproductive advantage, its frequency rises each generation.

Sources of heritable variation.

• Mutation — only source of new alleles; usually rare and random.
• Sexual reproduction — meiosis (crossing over + independent assortment) and random fertilisation generate vast new allele combinations.

(Acquired traits — e.g. a muscular build from training — are NOT inherited; only genetic changes pass to offspring.)

Examples.

Peppered moth (industrial melanism). Pre-Industrial Revolution: light-coloured moths blended with lichen-covered tree bark; dark variants stood out and were eaten. Industrial pollution killed lichens and darkened bark with soot → dark moths now better camouflaged; population shifted to dark. Reduction of pollution since 1970s reversed the trend.

Antibiotic resistance in bacteria. Mutations confer resistance to antibiotics. With antibiotic use, resistant bacteria survive and reproduce; resistant strains spread. Hospital "superbugs" like MRSA result.

Darwin's finches. Different islands of the Galápagos have different beak shapes adapted to different food (large seeds, small seeds, insects). Each species evolved from a single colonising ancestor — adaptive radiation.

Speciation. Process by which one species splits into two or more distinct species.

Reproductive isolation is the key requirement: populations must be prevented from interbreeding so divergent evolution can occur.

Allopatric speciation (geographic isolation):

1. A barrier (mountain, river, sea-level change) separates a population.
2. Different selection pressures + different mutations on each side.
3. Over many generations, the populations diverge.
4. Even if barrier is removed, they no longer interbreed → separate species.

Example: Darwin's finches.

Sympatric speciation (same geographic area):

• Reproductive isolation arises without physical separation — e.g. polyploidy in plants, temporal isolation (different breeding times), behavioural isolation (different mating displays).
• Less common in animals than allopatric.

Reproductive isolating mechanisms:

• Pre-zygotic — prevent mating or fertilisation (different breeding seasons, different songs, incompatible reproductive structures).
• Post-zygotic — hybrids are sterile (mule) or inviable.

Adaptive radiation. Rapid speciation of a lineage to fill many ecological niches — Galápagos finches, Hawaiian honeycreepers, African cichlids (~500 species in Lake Malawi).

Evolution is observable. Natural selection has been documented in real time — e.g. beak depth in Galápagos ground finches changed measurably between drought and rainy years (Peter and Rosemary Grant's decades-long study).