I missed Dienekes’s post when the debate on Ashraf and Galor’s paper on genetic diversity and economic growth was going strong, but he makes some interesting points on whether genetic diversity would be positive or not.
Genetic diversity is, in general, a good thing for a population, for a simple reason: adaptation via natural selection depends on the existence of variation (there cannot be selection in the absence of alternatives). Other things being equal, a population possessing a greater amount of genetic diversity has a greater probability of already possessing adaptive alleles that might be necessary to meet new environmental challenges (e.g., pathogens).
Of course, for the relevant genes, natural selection eliminates the diversity. In the long-run, we generally won’t see diversity in the most strongly selected traits. Dienekes continues:
But, we must also remember that genetic diversity can be partitioned to what is useful, neutral, or deleterious. We ought to be thankful that our Major Histocompatibility Complex (MHC) region is diverse, largely indifferent whether neutrally involving microsatellite loci have higher or lower allele variance in two populations, and a little concerned if there is an abundance of mildly deleterious rare variants sprinkled in our genomes, or a strong-effect disease-causing variant in one locus. …
And, indeed, even the boundaries between the useful/neutral/deleterious categories are blurred. Deleterious anaemia-causing mutations are known to have benefits of malaria-resistance. Neutral variants may be “useful” ones in waiting: for example, lactase persistent mutants may have existed in the human species for hundreds of thousands of years, appearing and re-appearing by mutation, but it is when they encountered cow’s milk and the need to drink it that they shifted from “neutral” to “useful”. And, even useful alleles can cease to be so, e.g., the eradication of swamps and malaria in Greece has removed the benefit of malaria-resistence, and left only the harm of anaemia.
The measures of genetic diversity used by Ashraf and Galor are based on non-protein coding regions of the genome, so may be neutral. However, the genetic diversity measured using these non-protein coding regions may be reflected in the functional diversity that was initially available to populations when the new populations were established.
It raises the question of what form of genetic diversity would be useful in a population. In many qualitative traits such as intelligence, diversity would not seem to be helpful. Yet an economy with significant specialisation and trade is likely to have roles for a broad range of people.
Dienekes also questions whether the genetic diversity in today’s populations is the result of the Out-of-Africa event.
Evidence has been slowly and steadily accumulating, that people who live in different parts of the world today are not necessarily the same people of the ones who lived there a few thousand years ago. Migration and admixture have changed the landscape of human genetic variation: migration by expanding “narrow” genetic pools into much wider territories, and admixture by increasing diversity in contact zones.
I do not see this point as being fatal for the genetic diversity and economic growth hypothesis, as migratory distance from Africa is correlated with genetic diversity even with localised migration and admixture. Perhaps as understanding of migrations and admixture is developed, there may be potential for more refined analysis of hypotheses such as Ashraf and Galor’s.