larger African population. Thus, the opsin polymorphism argues (mildly) against a complete replacement of the Caucasian gene pool by African populations.

The second example concerns an autosomal recessive disorder in lipid metabolism due to the absence of apolipoprotein C-II, the physiological activator of lipoprotein lipase, a key enzyme in very low density lipoprotein metabolism. Two deleterious alleles, one from a Venezuelan Caucasian family and one from a Japanese family, share a frameshift mutation suggesting common ancestry (Li et al., 1993). These two mutants diverged from the normal allele at least 2 Myr B.P. (Li et al., 1993). The persistence of two defective alleles over such a long time is a puzzle, perhaps a consequence of small heterozygote advantage. But this persistence (i) argues against extremely small population bottlenecks throughout Pleistocene human history, and (ii) favors the conclusion that European and Asian H. erectus have contributed to the gene pool of modern H. sapiens (Li et al., 1993).

Summary

The major histocompatibility complex (MHC) plays a cardinal role in the defense of vertebrates against parasites and other pathogens. In some genes there are extensive and ancient polymorphisms that have passed from ancestral to descendant species and are shared among contemporary species. The polymorphism at the DRB1 locus, represented by 58 known alleles in humans, has existed for at least 30 million years and is shared by humans, apes, and other primates. The coalescence theory of population genetics leads to the conclusion that the DRB1 polymorphism 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 exclude 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. The data are consistent with, but do not provide specific support for, the claim that human populations throughout the World were at that time replaced by populations migrating from Africa. The MHC and other molecular polymorphisms are consistent with a "multiregional" theory of Pleistocene human evolution that proposes regional continuity of human populations since the time of migrations of Homo erectus to the present,



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