Natural Selection · Paper 2 — Free Biology SL Practice Questions

The peppered moth (Biston betularia) is a classic example of natural selection. Before industrialisation, most peppered moths in England had light, speckled colouration (typica morph); a dark melanic morph (carbonaria morph) was very rare. During the Industrial Revolution, soot from factories darkened tree trunks and killed light-coloured lichens. Bird predation experiments showed that on soot-darkened trees, typica moths were eaten more frequently than carbonaria moths. - Frequency of carbonaria morph in Manchester in 1848: $2\%$ - Frequency of carbonaria morph in Manchester in 1895: $98\%$ - Generation time of peppered moth: $1$ year

A population of cheetahs (Acinonyx jubatus) lives in a nature reserve in Namibia. A researcher measures the running speed of $200$ adult cheetahs. The fastest $10\%$ of cheetahs (those running over $110\,\text{km\,h}^{-1}$) produce, on average, $4$ surviving cubs per year. The slowest $10\%$ (those running under $85\,\text{km\,h}^{-1}$) produce, on average, $1$ surviving cub per year. The remaining $80\%$ produce, on average, $2$ surviving cubs per year. Running speed is a heritable, polygenic trait.

The peppered moth (Biston betularia) is a classic example of natural selection driven by industrial pollution. In 19th-century England, two phenotypes existed: light-coloured (typica) and dark-coloured (carbonaria). Before industrialisation, typica moths were camouflaged against lichen-covered bark. After pollution darkened tree trunks with soot, carbonaria increased in frequency. Observed frequencies: Period — carbonaria frequency — typica frequency Pre-industrial (1848) — $2\%$ — $98\%$ Post-industrial (1895) — $95\%$ — $5\%$ Relative fitness values ($w$): Phenotype — Polluted forest — Unpolluted forest carbonaria — $0.95$ — $0.40$ typica — $0.50$ — $0.90$ Generation time of Biston betularia: 1 year.