Species and population

A population is a group of individuals of one species living in a definite area within the species range, freely interbreeding and relatively isolated from other such groups. A species consists of populations, so it is polytypic. The population is the elementary unit of evolution: evolution proceeds through changes in the gene pool (the sum of all alleles) of populations, not of individuals. A population is described by its number, density, age and sex structure, and birth and death rates. The gene pool changes under mutations, random loss or increase of alleles (genetic drift – stronger in small populations, studied by S. Wright), population waves (periodic changes of number), gene flow and natural selection. Isolation stops gene exchange between populations and lets differences build up: geographic (river, mountain, sea), ecological (different environments), biological (different breeding times or reproductive organs) and ethological (different behaviour). Allele frequencies in a population satisfy p + q = 1; by the Hardy–Weinberg law, in an ideal population the genotype frequencies are p² + 2pq + q² = 1 and do not change between generations.