The subject of bottlenecks in ancient human populations is visited once again, as Amos and Hoffman propose to have found evidence for two such events, one as humans migrated out of Africa and later when a migration event into Pleistocene America occurred across the Bering Strait.
Here’s the abstract of the paper which is freely accessible:
There is a strong consensus that modern humans originated in Africa and moved out to colonize the world approximately 50 000 years ago. During the process of expansion, variability was lost, creating a linear gradient of decreasing diversity with increasing distance from Africa. However, the exact way in which this loss occurred remains somewhat unclear: did it involve one, a few or a continuous series of population bottlenecks? We addressed this by analysing a large published dataset of 783 microsatellite loci genotyped in 53 worldwide populations, using the program ‘Bottleneck’.
Immediately following a sharp population decline, rare alleles are lost faster than heterozygosity, creating a transient excess of heterozygosity relative to allele number, a feature that is used by Bottleneck to infer historical events. We find evidence of two primary events, one ‘out of Africa’ and one placed around the Bering Strait, where an ancient land bridge allowed passage into the Americas. These findings agree well with the regions of the world where the largest founder events might have been expected, but contrast with the apparently smooth gradient of variability that is revealed when current heterozygosity is plotted against distance from Africa.
The researchers suggest that their more detailed approach to investigating the data allows for a more complex picture to emerge, which in the process threw up some unexpected findings, as revealed towards the end of the paper:
Despite these complications, a rather consistent pattern emerges, with evidence of a bottleneck being strongest in the Middle East and in the easternmost East Asian/northernmost American populations. These two locations are as one might expect, but there are two additional features that are less obvious. First, the African populations, although at most loci having low t-values, do provide quite strong and consistent evidence of a bottleneck at the lowest variability loci. As discussed, this may reflect an observation bias in which loci with very low variability in Africa are unusual for some reason other than demography.
An alternative explanation is that these loci still retain the signal of an even more ancient, within-Africa event. This would be consistent with the notion that locus variability is inversely related to the antiquity of the bottleneck signal that is best retained and offers an intriguing hypothesis for future studies. The second feature is the pronounced dip in t-value between Europe/central southern Asia and East Asia. This may simply reflect a null signal between two bottlenecks, but might alternatively indicate some other demographic event such as a period of stasis and population expansion. Again, further work is desirable.
Evidence that two main bottleneck events shaped modern human genetic diversity by W. Amos and J. I. Hoffman, 2009 – Published online before print October 7, 2009, doi: 10.1098/rspb.2009.1473