Saturday, October 29, 2005

When human biodiversity matters   posted by Razib @ 10/29/2005 06:50:00 PM

A few weeks ago I finished reading 1491 by Charles C. Mann, which I haven't said much about because it didn't tell me much new (i.e., there were many more natives than we had earlier thought, the Clovis culture wasn't here first, Amazonia shows strong evidence of being an anthropogenically shaped environment). I might comment in detail at some point, I have a book on Maya history that I want to read at some point that might give me some ideas that are bloggable, but, I want to highlight a passage in the book where the author talks to a geneticist in regards to native susceptibility to Old World pathogens:

...I again telephoned Francis Black. Being genetically determined, Indian HLA homogeneity cannot be changed (except by intermarriage with non-Indians). Did that mean that the epidemics were unavoidable...Could the mass death have been averted?

"There have been lots of cases where individual towns kept out epidemics...."

He stopped speaking for long enough that I asked him if he was still on the line.

"I'm trying to imagine how you would do it...."
"You'd have to have the entire hemisphere doing that. And the Europeans would presumably have to cooperate, or most of them, anyway. I can't imagine that happening, actually. Any of it."

As I have noted many times before, the HLA loci, which are crucial to our adaptive immune system, are undisputed cases where genetic diversity has been crucial to long term fitness. Some of the HLA alleles coalesce before the chimp-human speciation event, you might share some alleles with chimpanzees, but not with your own siblings! If a mutation is neutral, its chance of fixation is 1/2Ne, where Ne is the effective breeding population. In other words, the larger the population the less likely that a mutation is likely to random walk in frequency until fixation, but given enough time, there will always be turnover. If the mutation is favorable the probability of fixation is 2s, where s is the selection coefficient which defines the fitness advantage confered by the derived allele against the population mean fitness. Again, given enough time one imagines that one favored allele should fix itself within a breeding population. The deep time persistence of numerous alleles on these loci in most world populations suggest either heterozygote advantage or frequency dependent selection (where lower frequency would increase fitness).

Most of you probably know why this is relevant for native peoples of the New World: they exhibit relatively little HLA diversity, and, the frequencies of their modal alleles are often rather high. There might be many reasons for this, not least of which was the likely population bottleneck most of their ancestors went through. Additionally, Mann reports that some researchers are finding that the helper-T cells of native peoples seems to be biased toward resisting parasites, rather than against microorganisms, as is the case amongst Europeans (one reason might be the relative lack of domesticated animals to incubated species jumping pathogens).

This isn't particularly new information, and normally I wouldn't repeat it, but do for two reasons. First, a few weeks ago an acquaintance of mine expressed skepticism that populational level genetic differences could have any relevance for the human sciences. In this case, the arc of world history was crucially affected. But the second point is that the highbound population figures for the New World implies that within 200 years between 1500-1700 the human population on this planet might have declined by as much as 20%. We are generally aware that rabbits were an invasive species which swarmed across a vulnerable ecosystem in Australia, or flightless birds were doomed to extinction in the face of rats and cats. And yet I had never paused and reflected that evaluated on the species level the short term impact of the contact between the Old World and New was so devastating for our numbers! Granted, I am personally skeptical of the highbound numbers (on the order of 100 million) given as a population base for the New World, but even more conservative numbers are sobering (the 2-4 million range seems highly implausible to me).

It is a sobering reminder that reality does not brook our moral sensibilities, and I can't but help chuckle at some of the more benign renderings of the Strong Anthropic Principle.

Related: Here is a precis of Mann's book in The Atlantic.