Genetic effects of human migrations and isolation

Abstract

This paper analyses how migrations, environment and epidemics interact to shape genetic variation in the moder human species.

The gene mutation that makes humans resistant to malaria is a striking example of how disease can shape the human genome. In Europe malaria spread in coincidence with the arrival of populations from Asia Minor and eastern Mediterranean and was favoured by the spread of agriculture, by the sedentary conditions of life and the related demographic increase.

Natural selection, generally, shape the gene pool of a population in order to fit a different environment. This is the reason because hemoglobinopathies and enzyme G6PD deficit are greatly spread in areas hit by malaria epidemic. These effects are particularly evident in isolated regions or in islands with low population density, e.g. Sardinia.

Disasters such as epidemics may drastically reduced the size of a population, and the victims under such circumstances are not selected. As a result the survivors within this small population are unlikely to be representative of the original population in its genetic makeup, and this occurrence is known as “bottleneck effect”. Sardinia, for instance, was hit between 1300 and 1700 by several plague epidemics. Such events drastically reduced the total number of inhabitants; creating a local alteration in the gene frequencies, that have moulded the genetics of the population. This has brought about not only a differentiation with respect to other Mediterranean populations, but creating a variability inside the island.