Below, in the Celts & Iberians entry, there was some talk about Basques and their genetics. There is one thing to keep in mind about Basques genetically, which can be illustrated with this table:

Since Rh- negative mothers often have an adverse reaction to carrying babies who are Rh+ (an immune reaction), miscarriage would be a common result (this is before modern medicine). This is a clear case of a genetic barrier in terms of fecundity in cross-population matings. I recall reading a book on Basque history about 5 years ago where it was observed by their neighbors that the Basque women did miscarry more than was usual, and that this was perhaps a sign from God at the disfavor he held them in. There are differences on other locii, this paper suggests that "Significant population differentiation was registered between Basques and all the other Iberian populations and also between Valencia and Northern Portugal." This makes sense since one would except the male lineage to be preserved as Basque elites resist the spread of alien languages. This paper suggests that the Basques also differ somewhat on the mtDNA (female) lineage. But my impression is that NRY and mtDNA lineages are not as variant as the frequency of Rh-. Here is Henry Harpending on the possibility of selection for Rh- among the Basques (source):

Their genes are mostly similar to those of their neighbors except that they have a frequency of Rhesus negative that is greater than one-half. Following Ruhlen (1994) imagine that there has been some small level of gene flow, one percent per generation, into the Basque population from their new neighbors for five thousand years, say 250 generations. Then of the neutral genes in the Basque population today only .99²⁵⁰ eight percent are descended from ancient Basque genes. The neutral parts of the genome have essential been replaced.

But consider the languages spoken by these immigrants. Since they arrive in small numbers they and their children learn Basque and, save for occasional loan words, have little effect on the language. The Basque language has persisted over millenia while the neutral genome has been replaced. Meanwhile, natural selection at the Rh locus is such that the common type is favored. If the original state was all or mostly Rh negative, then there would ongoing selection against any Rh positive genes introduced by the immigrants. In this way, both language and the Rh system preserve deeper history than neutral genes. Rh and language share the property that there is selection for the common type. Sexual selection for external appearence may follow similar dynamics.

Did you follow that? Note that even if the Basque had lower average fecundity because of the their high Rh- frequencies, it might have served to shore up ingroup cohesion if Basque + Basque matings were more likely to be fecund than Basque + non-Basque matings because of Rh incompatibilities.