The theory of genetic distance and evolution of human races
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Theoretical works on Nei's genetic distance and its extensions are discussed. New formulae for the sampling variances of genetic distance estimates are presented. Formulae for the genetic identity of genes at the electrophoretic level when the mutation rate varies from locus to locus are also presented.
Empirical data suggests that the rate of gene substitution or mutation rate per locus varies considerably among protein loci, and if this factor is taken into account, the rate of decline of genetic identity (I) is no longer constant but decreases with evolutionary time. Using both the infiniteallele model and the stepwise mutation model, the numerical relationship betweenI and evolutionary time is presented. This relationship may be used for estimating the time after divergence between populations. The value of genetic distance or genetic identity is also affected considerably by the bottleneck effect. The bottleneck effect generally accelerates the increase of genetic distance with time, and the effect remains for a long time after the population size returns to the original level. A method for correcting for this effect is presented.
Application of the theory of genetic distance to data on protein polymorphism in man indicates that the genetic variation between the three major races, Caucasoid, Negroid, and Mongoloid, is much smaller than the variation within them, despite the fact that there is a conspicuous difference in some morphological characters such as pigmentation, facial structure, and hair texture. It is proposed that the differentiation of these morphological characters was brought about by relatively strong natural selection through a small number of gene substitutions, whereas general protein loci are subject to little or very weak selection. Analysis of blood group gene frequency data gives essentially the same result as those from protein loci, though they are likely to have been affected by nonrandom sampling of the loci. It is also shown that at the protein level the racial differences in man correspond to those between local races in other organisms.
Rough estimates of the number of codon differences between an individual of man and his various relatives are presented. It seems that the mean number of codon differences between man and chimpanzee is about 10 times larger than that between second degree relatives in Caucasians or Japanese, but about 1/19 of that between man and horse.
Genetic distance estimates suggest that among the three major races of man the first divergence occurred about 120,000 years ago between Negroid and a group of Caucasoid and Mongoloid and then the latter group split into Caucasoid and Mongoloid around 60,000 years ago. It is also shown that the genetic identity between man and chimpanzee corresponds to a divergence time of 4–6 million years if the assumption of constant rate of amino acid substitution is correct.
Methods of constructing a phylogenetic tree from genetic distance estimates are discussed. For constructing the topology of a tree, Fitch and Margoliash's method is quite efficient. For estimating branch lengths, however, Nei's method of averaging distances seems to be better.
A phylogenetic tree for twelve races of man is constructed by using gene frequency data for 11 protein and 11 blood group loci. This tree roughly agrees with what we expect intuitively from the morphological characters and the historical record of these races.
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