>Glenn wrote:
>>True, but this is really nitpicking. Genetics neither supports a recent Adam/Eve nor a Genetic bottleneck of the magnitude of the Biblical Flood. Why? There are up to 107 alleles found in certain genes in the major histocompatibility complex. Here is what one study of the DRB1 site in the MHC showed. It compared us with other animals and found common alleles with other critters--so much so that the authors concluded: "The coalescence theory of population genetics leads to the conclusion that the DRB1 polymophism requires that the population ancestral to modern humans has maintained a mean effective size of 100,000 individuals over the 30-million-year persistence of this polymorphism. We explore the possibility of occasional population bottlenecks and conclude that the ancestral population could not have at any time consisted of fewer than several thousand individuals. The MHC polymorphisms excluded the theory claiming, on the basis of mitochondrial DNA polymorphisms, !
that a constriction down to one or few women occurred in Africa, at the transition from archaic to anatomically modern humans, some 200,000 years ago." ~ Francisco J. Ayala, Ananias Escalante, Colm O'Huigin and Jan Klein, "Molecular Genetics of Speciation and Human Origins," Proc. Natl. Acad. Sci, USA, 91:pp6787-6794, July 1994, p. 6787.
>
>Question for any real population geneticists here:
>
>Is it possible that something like population level selection for polymorphism at these loci could throw this estimate off?
>
>Preston G.
I am not a real population geneticist, but I taught a bit of it this semester. Yes, it would throw off any estimate of timing, because such selective pressure (which is to be expected in many immune-system related genes, either on the part of the pathogen or the host) will increase the level of polymorphism. The problem comes in if there are more varieties that are also found in other organisms than are present in any one individual. In that case, more than one individual had to get
MHC genes from other organisms, most easily by being independently descended from them. Lateral transfer might be a factor. Another possibility might be that there is a limited range of functional options, so that the similar human and non-human sequences are convergent. As these are guesses off the top of my head without knowing the details of the MHC system, it's hard to tell if any of these ideas has much chance of correctness.
Dr. David Campbell
"Old Seashells"
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
dcampbell@osprey.smcm.edu, 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!
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