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The mating structure of early human populations, and its genetic consequences

PI(s): Alan Bittles (Murdoch University)
Start Date: 30-Mar-2012
End Date: 28-Apr-2012
Keywords: population genetics, mating systems, inbreeding, human evolution, biomedical

In present-day Western societies there is a general belief that marriage between close relatives is harmful, as evidenced by the fact that first cousin marriage is prohibited in 31 of the 50 states of the Union. Yet given the very small numbers of humans who were our ‘Out-of-Africa’ forebears, with estimates ranging from just 700 to approximately 10,000 individuals, it seems inevitable that some level of close kin mating would have occurred. The central question that arises is to what extent was this inbreeding harmful, but conversely to what extent might it have been helpful in selecting against disease mutations and thus preventing genetic disease? The process of negative selection is referred to as purging, and it has been claimed that in a number of populations, such as South India where some 20% of marriages are between uncles and their nieces, close kin marriages over successive generations have resulted in purging of the gene pool, resulting in fewer inherited diseases. Unfortunately, there has been little credible evidence to prove or disprove this theory, which is of considerable contemporary relevance since ongoing increases in human population sizes have led to population dispersal and a decline in close kin marriage. This shift away from cousin marriage has been widely interpreted in a positive light, with reductions in the prevalence of genetic disorders predicted. But if reduced purging of the gene pool is now occurring, could the predicted gains in population health prove to be short-term rather than long-term in their effect?