Sunday, December 30, 2007

Neandertal lovin'   posted by p-ter @ 12/30/2007 01:16:00 PM

John Hawks, Bruce Lahn, and company have a fun paper in Trends in Genetics on the role of introgression of adaptive alleles from archaic Homo species in human evolution. The key point:
[A]n allele with a 5% advantage has a 10% chance of fixation. In fact, selected alleles in an exponentially growing population have a slightly higher probability of fixation, augmented by twice the intrinsic rate of growth. Because each copy of an introduced allele has this fixation probability, only a small number of matings between archaic and modern populations would ensure the eventual fixation of a large proportion of the adaptive archaic alleles. For example, for any and all advantageous archaic variants with s = 0.01, a 95% probability of fixation requires only 74 archaic-modern matings, each introducing a single copy of the allele into the modern human population. Widespread introgression of selected alleles would occur with a minimal level of interbreeding, which would leave a negligible effect on even large samples of neutral loci.
There are, of course, assumptions in these sorts of caculations--perhaps most importantly, the fitness advantage of an introgressed allele would probably be canceled out or reversed by the effects of hybrid incompatibility loci for the first few generations-- but it certainly seems reasonable to assume that a number of alleles slipped through the species barrier.

The authors point to a number of candidate gene studies, and have some speculation about the types of alleles that might have been beneficial to the invasive Homo sapiens. To systematically find these alleles and be able to generalize about them, one needs large-scale genome sequencing. Luckily, this type of data is being generated, as I type, at genome centers across the world.