Natural Selection and Population Genetics

Variation may be discontinuous (distinct categories, few genes, e.g. blood group) or continuous (range, many genes plus environment, e.g. height). Natural selection: individuals vary; a selection pressure gives some a survival and reproductive advantage; they pass on their alleles, so allele frequencies change over generations. Stabilising selection favours the average; directional favours one extreme; disruptive favours both extremes. Genetic drift changes allele frequencies by chance, strongest in small populations (founder effect, bottleneck). Hardy–Weinberg: in a large, randomly mating population with no selection, mutation or migration, $p+q=1$ and $p^2+2pq+q^2=1$, letting you estimate allele and carrier frequencies. Artificial selection is selective breeding by humans.