Researchers from the University of Geneva try to address the somewhat complex problem of genetic flow across the Strait of Gibraltar, between SW Europe and NW Africa, in a new paper:
Mathias Currat et al., Human genetic differentiation across the Strait of Gibraltar. BMC Evolutionary Biology, 2010. Open access.
They analyze a number of genetic variables such as Y-DNA, mtDNA, blood groups ABO and Rh, and other antigen/antibody systems (HLA, MNS and GM) and produce a number of statistical analysis for them. However they acknowledge that their modeling on continuous, rather than punctual, gene flow, may have some difficulties providing clear-cut answers.
Still they do produce results that are in general favorable for the P scenario (flow since 20,000 years ago), excepting to some extent the Y-DNA, which is probably a secondary sex-biased element overall.
Above: The scenarios are described in page 10 and in the supplementary material. P is "Paleolithic" flow beginning 20,000 years ago (Ibero-Maurusian or Oranian culture) among small populations, N instead is "Neolithic" flow among larger populations. PN is intermediate and PNI is intermediate but also considering the Islamic expansion (though details are not provided).
They also produce results of quite greater affinity within each region (SW Europe and NW Africa) than between them, as expected.
Notice in this graph (and also fig. 3) how autosomal markers' curves and that of mtDNA (MT-HV1, in blue) are very similar, however while mtDNA tends to lesser homogeneity within regions, it strongly tends to higher homogeneity instead across the Strait. This I interpret as meaning greater affinity across the strait in the Paleolithic, before the primarily male expansions reflected in the Y-DNA, altered the scenario.
Sadly no graphs for each of the two regions is provided, what does not allow us to make a more detailed assessment, like whether the Y-DNA expansion happened at both coasts or not, and, if so, if they were fully comparable or somehow different. I say this because the apparent SW European origin of North African mtDNA H (c. 25%) detected by Cherni 2008, and probably other haplogroups like V and K, would be consistent with a more European-like population before the E1b1b1 expansion in the context of Capsian culture (in North Africa) which is probably associated also to the expansion of Afroasiatic languages. However I see no reason to support a similar male-biased expansion in SW Europe.
Let the authors conclude:
While contrasted conclusions were obtained by previous studies based mostly on single genetic loci, our study clarifies the role of the Strait of Gibraltar regarding its permeability to gene flow. Indeed, our multi-locus approach led us to take into account variations between loci when trying to infer past history of human populations around the Gibraltar area. We were thus able to show that the Y chromosome on one side, and HLA-DRB1 on the other side constitute two extreme cases of very strong and very weak (respectively) genetic differentiations between populations across the Strait. The lack of genetic differentiation for HLA-DRB1 is particularly interesting because it can be explained by balancing selection (with a coefficient of selection estimated here to be around 2%). Given the huge worldwide dataset available for this locus, a better understanding on the mechanisms of selection at HLA loci could be very helpful to the study of human evolution, and more generally MHC. Our results obtained for Gibraltar have to be confirmed by further studies in other areas, especially where gene flow between populations is reduced. This work thus constitutes a step forward towards a better characterization of the combined effects of selection and demography on the genetic structure of populations, and especially on their genetic differentiation.