D4.1 Natural selectionIB Biology HL: Revision notes

Natural selection is the mechanism of evolutionary change; it has acted continuously for billions of years, producing Earth's biodiversity. Darwin's theory supplied the mechanism that earlier naturalists lacked and replaced Lamarckism (inheritance of acquired characteristics) — a paradigm shift.

Variation comes from mutation (new alleles) and sexual reproduction (new combinations of alleles). Only heritable traits evolve: characteristics acquired during life are not encoded in the base sequence of genes.

Populations overproduce offspring, and resources such as food or nest sites limit carrying capacity, so there is intraspecific competition. Abiotic factors such as extreme temperatures act as density-independent selection pressures. Fitness is the survival value and reproductive potential of a genotype.

Sexual selection: traits that signal overall fitness (plumage of birds of paradise) increase mating success. Endler's guppies: female choice favours bright males; strong predators (pike cichlid) favour dull ones. Controlling predators in ponds or by transferring fish models these opposing pressures.

A gene pool is all the genes and their different alleles present in a population. Geographically isolated populations have no gene flow, so their allele frequencies can differ. Human example: the cystic fibrosis allele, or the lactase persistence allele, has very different frequencies in different populations; these can be searched in online allele-frequency databases.

Natural selection between individuals with different heritable traits changes allele frequencies in the gene pool. Darwin lacked genetics; neo-Darwinism integrates genetics with natural selection.

• Directional: one extreme is favoured; the mean shifts (e.g. antibiotic resistance).
• Disruptive: both extremes are favoured over intermediates; can split a population.
• Stabilizing: intermediates are favoured and extremes removed; variation narrows (e.g. human birth mass).

All three change allele frequencies.

For a gene with two alleles, p and q are the allele frequencies and p + q = 1. With random mating, genotype frequencies are p² + 2pq + q² = 1 (p² = homozygous dominant, 2pq = heterozygous, q² = homozygous recessive).

If one genotype frequency is known, work back: e.g. 1 in 2500 affected by a recessive condition → q² = 0.0004, q = 0.02, p = 0.98, carriers 2pq = 0.0392 (about 1 in 25).

Genetic equilibrium requires: a large population, random mating, no migration (gene flow), no mutation and no selection (equal survival and reproduction of genotypes). If observed genotype frequencies do not fit the equation, at least one condition is not being met — for example mating is non-random or survival varies between genotypes.

Artificial selection is the deliberate choice of individuals with desirable traits for breeding, in crop plants (cabbage → broccoli, cauliflower, Brussels sprouts) and domesticated animals. Unintended consequences of human action, such as antibiotic resistance in bacteria or herbicide resistance in weeds, result from natural selection, because nobody chooses which individuals breed.