Evolution & Natural Selection – A-Level Biology Revision Notes

Natural selection is the process by which individuals with advantageous alleles are more likely to survive, reproduce and pass on those alleles. Evolution is the result: a change in the allele frequencies of a population over many generations. In short, natural selection is the mechanism, and evolution (the change in the gene pool over time) is the outcome. It is the population that evolves, not the individual.

In a population of bacteria, a random mutation gives some individuals resistance to an antibiotic. This mutation exists before the antibiotic is used. When the antibiotic is applied it acts as a selection pressure, killing the non-resistant bacteria while the resistant ones survive and reproduce. The resistance allele is passed on, so its frequency increases in the population over generations. The key point examiners want is that the mutation came first and the antibiotic did not cause it.

Directional selection favours one extreme of a range, so the mean shifts towards it over time (for example antibiotic resistance). Stabilising selection favours the intermediate phenotype and selects against both extremes, reducing variation and keeping the mean the same (for example human birth weight). Disruptive selection favours both extremes against the middle, increasing variation and potentially splitting a population. You should be able to identify each type from a graph of phenotype frequency.

Use p + q = 1 for allele frequencies and p² + 2pq + q² = 1 for genotype frequencies, where p is the dominant allele frequency and q is the recessive. The usual route is to start from the recessive phenotype: its frequency equals q², so take the square root to find q, then use p = 1 − q to find p. From p and q you can calculate p² (homozygous dominant) and 2pq (heterozygous carriers). A common mistake is confusing q with q² — the fraction showing the recessive condition is q².

The Hardy–Weinberg principle assumes a large population, no mutation, no migration (no gene flow into or out of the population), no natural selection (all genotypes are equally fertile), and random mating. If these conditions hold, allele frequencies stay constant from generation to generation. If allele frequencies do change, one of the conditions has been broken — which means the population is evolving. Examiners often award a mark for stating these conditions.

Genetic drift is a change in allele frequency caused by random chance rather than by natural selection. It has a much greater effect in small populations, where chance events — such as which individuals happen to breed — can change allele frequencies dramatically between generations. The founder effect (a small group starting a new population with only a sample of the original alleles) and genetic bottlenecks (a population crashing to a small size) are both examples of genetic drift.

Both produce new species through reproductive isolation, but the cause differs. Allopatric speciation happens when populations are separated by a geographical barrier such as a river, mountain or ocean, so gene flow stops and the populations diverge. Sympatric speciation happens within the same area, without a physical barrier, through reproductive isolation such as different mating seasons, behaviours or mechanical incompatibility. In both cases the isolated gene pools accumulate different changes until the populations can no longer interbreed to produce fertile offspring.

A species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring. The word fertile is essential: two organisms that can mate but produce sterile offspring (such as a horse and a donkey producing a mule) are not the same species. This biological species concept underpins speciation — a new species has formed once two populations can no longer interbreed to produce fertile offspring.