Demic Expansions and Human Evolution

Abstract

Geographic expansions are caused by successful innovations, biological or cultural, that favor local growth and movement. They have had a powerful effect in determining the present patterns of human genetic geography. Modern human populations expanded rapidly across the Earth in the last 100,000 years. At the end of the Paleolithic (10,000 years ago) only a few islands and other areas were unoccupied. The number of inhabitants was then about one thousand times smaller than it is now. Population densities were low throughout the Paleolithic, and random genetic drift was therefore especially effective. Major genetic differences between living human groups must have evolved at that time. Population growths that began afterward, especially with the spread of agriculture, progressively reduced the drift in population and the resulting genetic differentiation. Genetic traces of the expansions that these growths determined are still recognizable.

Frequency gradients of DHCR7 mutations in patients with Smith-Lemli-Opitz syndrome in Europe: evidence for different origins of common mutations

“…The presence of opposite gradients from west to east and east to west of several DHCR7 mutations is not easily explained by the demic diffusion model of the peopling of Europe by neolithic farmers, which is believed to explain some of the gene frequency gradients in Europe (first principle component). 28,29 Rather, a more complex situation seems to exist. Among the populations studied the frequency of the IVS8-1G > C mutation is highest in Britain.…”

Human X-chromosomal lineages in Europe reveal Middle Eastern and Asiatic contacts

“…42 Clines are an expected consequence of major population movements in which population expansion into new territories is accompanied by repeated founder effects and subsequent population growth. 43,44 To be able to cause such a significant cline, it is necessary that the population expansion happened when population numbers were relatively small, and hence an expansion would be able to have a major effect on allele frequencies across a whole subcontinent. This reasoning, in accordance with the fact that the dys44 marker system has an increased time depth to that of mtDNA and Y-chromosome data, leads us to the conclusion that the demographic events that we witness through the cline were ancient ones (pre-Neolithic).…”

Genetic variation, classification and 'race'

“…Because of a history of extensive migration and gene flow, however, human genetic variation tends to be distributed in a continuous fashion and seldom has marked geographic discontinuities 19,42 . Thus, populations are never 'pure' in a genetic sense, and definite boundaries between individuals or populations (e.g., 'races') will be necessarily somewhat inaccurate and arbitrary.…”