Selection and evolution
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, and , letting you estimate allele and carrier frequencies. Artificial selection is selective breeding by humans.
≈ 50 minSpeciation and evolution
A species (biological concept) is a group of organisms that can interbreed to produce fertile offspring. Speciation requires reproductive isolation, which stops gene flow. Allopatric speciation: a geographical barrier (river, mountain, sea) separates populations; different mutations, selection pressures and genetic drift make their allele frequencies diverge until they can't interbreed. Sympatric speciation happens in the same area through ecological, behavioural or temporal isolation, or polyploidy in plants. Isolating mechanisms may be prezygotic (different breeding seasons, courtship, incompatible genitalia) or postzygotic (hybrid inviability or sterility, e.g. mules). Molecular evidence (DNA base sequences, mitochondrial DNA, amino acid sequences of proteins such as cytochrome c) shows relationships: fewer differences mean more recent common ancestry.
≈ 45 min