Is genetic diversity good or bad?

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.

7 comments

  1. “In many qualitative traits such as intelligence, diversity would not seem to be helpful.”

    I would argue the opposite. A wide distribution of IQ is likely adaptive. There is no need for a society full of Einsteins. It is more than sufficient if one or a few members of the tribe are intelligent and the others develop other useful skills.

    Intelligent people commit suicide more often. Academics reproduce less, at least in Europe. Some time ago, a science radio program (Leonardo, WDR5 in NRW, Germany) presented some numbers on the average number of kids per scientist. I forgot the number, but it was extremely low. The presenter wryly added: if it were a zoo, people would complain about bad housing and diet (keine artgerechte Tierhaltung) with such a fertility rate.

  2. Some thoughts on “In many qualitative traits such as intelligence, diversity would not seem to be helpful.”

    One way to think about general intelligence is that is involves having a large tool-kit of cognitive behaviors which can be mixed and matched for complex thinking. Corvids, parrots, and humans appear to have had large gene pools over a wide variety of habitats for long enough periods of evolutionary time to have resulted in large tool-kits of cognitive behaviors. A temporal bottleneck seems likely around the problem of having many mental modules and yet having sufficient selection pressure on those modules to maintain their functionality. As the old saying goes — natural selection uses up genetic variation. (For mental modularity debates see papers/writings by Robert Kurzban and by H. Clarke Barrett.) Coevolutionary-races for example push species toward every increasing niche specialization, working against the outlier cases where species carry large tool-kits. (Hence null SETI results, by the way.) So the level of granularity for thinking about the usefulness of diversity may be better conceptualized for some purposes at the level of traits than at alleles. (Of course selection-events are trait-environment interactions and genes convey the results of those events.) .. with the resulting argument that diversity is the ~foundation~ of general intelligence, or at least this particular sense of general intelligence being used in the animal behavior literature.

    1. To the extent you can break intelligence into different components, I would agree. But if we take any of those components, I am not sure that, from the perspective of economic development, having more variation would be better than having a higher level. Adaptively there may be advantages to the variation, particularly as large brains are costly, but economically I am not so sure.

      1. Yes, nowadays there is clearly a economic premium on intelligence and education.

        Although, even today you can have too many academics, many are doing jobs well below their intellectual abilities.

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