Evolutionary forces in human populations in an environmental gradient in Arica, Chile.

Abstract

ACCORDING to the Hardy-Weinberg demonstration, in a large population gene frequencies are inherently stable, in the absence of the systematic modifying processes of migration, mutation and selection. But this property of stability does not hold in a small population, and the gene frequencies are subject to random deviation arising from the sampling of gametes (Wright, 1931). Thus, there occurs nonadaptive differentiation which may lead to fixation. Since this is random, the combination of genes fixed will not be that most favored by selection, and the fate of the population may be extinction (Li, 1955). In discusing the biological evolution of populations Li (1955) has point out that, according to Wright (1931) the most favorable condition for evolutionary change is that in which a large population is subdivided into numerous partly isolated groups with migration between them. In this case, systematic and dispersive evolutionary forces may be at work simultaneously, and in the interaction of these forces, dispersive processes are depressed, and complete fixation of genes and extinction will be prevented. In these conditions, the population can meet environmental changes with its diversified and flexible subgroups and will be able to develop adaptive norms in relation to the new situation.