The interpretation of the temporal and spatial variation of gene frequencies in populations is one of the main aims of population genetics. In addition to systematic evolutionary processes (mutation, selection and admixture), non-systematic factors such as size, density, and the distribution of a population, the nature of mate selection within a population, and differential fertility and viability also influence the genetic composition of a population. These non-systematic factors are the most important components of the structure of human populations. It is now well known that very different patterns of these non-systematic factors exist in human populations.