Looking for signatures of sex-specific demography and local adaptation on the X chromosome

See related research article: http://genomebiology.com/2010/11/1/R10 webcite.The genetic diversity of humans results from the interplay of three main types of evolutionary force. Mutation and recombination act directly on the genome and produce changes. These genetic changes may then be subject to selection, which can take different forms and leave various signatures locally on the genome. Finally, demographic forces such as migration and genetic drift affect the genome as a whole. Untangling the extent to which each of these forces has an impact on our genomic diversity is the focus of human evolutionary genetics.Thanks to the large amount of data now available, the number of studies focusing on this problem has grown exponentially over the past few years. A new and promising approach, used by Casto et al. [1] in a study published recently in Genome Biology, is to compare the genetic diversity of the X chromosome with that of the autosomes. This approach can reveal the importance of both demography and selection in shaping human genetic diversity. The X chromosome undergoes more drift than autosomes, as its effective population size (Ne) is three-quarters that of autosomes. The Ne for any population represents the size of an idealized, randomly mating population that experiences the same amount of genetic drift as the one under study. In particular it assumes an equilibrium sex ratio, a variance of reproductive success equal to 1 and no fluctuation of population size over time. The X chromosome is also asymmetrically influenced by male and female demographies, and it is strongly affected by selection, as recessive mutations are very efficiently exposed to selection in haploid males. The X chromosome therefore has the potential to reveal new aspects of the evolution of our genome.The first task of population genetics is to describe the distribution in the human genome of that part of our genetic diversity that arises solely from the interplay of the genomic and demo