The overriding questions concern how species arise and how species remain separate. The Hardy-Weinberg equation is an algebraic statement that can be used to predict allelic frequencies with populations, assuming certain constants are maintained. Hardy-Weinberg conditions are rarely, if ever, met and, as a result, evolution occurs. Nonrandom mating, an absolute requirement for Hardy-Weinberg equilibrium, does not occur in human populations.
Gene frequencies can change when a small group of individuals separates from a larger population.
This occurs when a new settlement is formed. This occurs when many members of a population die out and the remaining members mate to restore the population. This is a type of stabilizing selection. Natural selection, genetic drift, migration, nonrandom mating, mutation, and biogeography influence speciation. The organisms mate but do not produce fertile offspring.