David B at Gene Expression criticizes Frank Salter's contention that intra-ethnic marriage raises the fitness of a person's genes. Here is a small commentary on this matter:
Intra- or inter-racial marriage does nothing for the fitness of individual genes in itself, so anyone who chooses one or the other with the hope of promoting his ethnic genes is misguided.
If immigration is to be resisted it is because it shifts the resource/population ratio for different ethnic groups. An ethnic group which maximizes its resource/population ratio (e.g., handful of Mongols controlling the greater part of Eurasia) will eventually raise its fitness.
Additionally, widespread practice of inter-racial marriage may not (in itself) say much about shifts in gene frequencies, but it does say something about the potential for such shifts.
If the world consisted of 50% of group A and 50% of group B, then group A can hope to outplay group B and grow to e.g., 75% vs. 25%, and this would be beneficial for its specific alleles.
By incorporating these alleles in inter-racial offspring, the potential that these alleles will become more (or less) frequent diminishes.
If, for example China and India remain distinctive gene pools, then it is possible that sometime in the future China might nuke India or vice versa, and thus give a big frequency gain for the specific Mongoloid genes in the Chinese population. If, on the other hand there was widespread intermarriage between Indians and Chinese, the potential for such a change would diminish, since e.g., future warfare between the two countries would not affect the frequency of these genes to the same extent, as they would be present at appreciable frequencies in both territories.
So, ethnic groups who hope to propagate their distinctive genes should better maintain some level of genetic individuality - knowing of course that in this way they also risk the fate of countless extinct peoples. Historically, many peoples have taken this gamble, and some (like Europeans in North America) have succeeded, while others (like the Samaritans) have failed.