Genetic drift and polygenic inheritance.

The interaction of random gene drift and selection was studied by computer simulation for two quantitative traits, which were considered to approximate stature and skin color dlfferences in human populations. The expected effects of gene drift, fixation of alleles and reduction of genotypic and phenotypic variances, were found in the simulation. Stabilizing selection, which seems to be the type of selection operating on these traits, was found to increase the effects of gene drift. Since there seems to be no evidence of reduction in phenotypic and presumably genotypic variability in small human populations, the applicability of these simple genetic models to human traits raises problems for which several possible solutions exist. Ever since the development of the theory of population genetics over 50 years ago, its implications for the variability in metric traits among human populations has been a continuing problem. Most of the concepts of population genetics pertain to traits controlled by a single locus with a limited number of alleles, while most anthropometric traits are influenced by several loci and also by the environment. These complications must obviously be considered in any interpretation of the effects of mutation, selection, gene flow, and gene drift on metric traits. In particular, random genetic drift is known to change gene frequencies significantly in small populations, but the question remains as to its effect on anthropometric traits. Some time ago Birdsell (‘50) pointed out that, assuming the common polygenic model of equal and additive effects for all loci, one would expect very little change in the mean of any trait since the alleles increasing its size would be expected to increase and become fixed in the population as frequently as those decreasing the trait. However, selection would presumably have some effect on this process of drift, and its effect would obviously depend on how selection operated on the trait. Beginning with the classic paper by Fisher (‘18) on the correlation of metric traits among relatives, population genetics AM. J. PnYs. ANTHROP., 37: 117-126. theory has been applied to quantitative inheritance. The theory has been developed from and widely used in animal and plant breeding, but there have been few applications to the problems of human evolution. To a great extent this is obviously due to a lack of knowledge of both the mode of inheritance of anthropometric traits and the action of natural selection on them, but I think i t is also due to a lack of confidence in the applicability or usefulness of the simple models of quantitative inheritance. Certainly the concept or assumption of equal and additive loci is a crude approximation and the number of loci affecting most metric traits is not known, but the importance of these assumptions to the evolutionary process will only become known through a comparison of our crude models with data. The models of quantitative inheritance are also so complex that it has only been since the development of the computer that any attempt could be made to answer some of the questions. Formal solutions are frequently not possible, but simulations with various sets of numerical estimates for the many parameters can provide some idea of the effects of the various forces of evolution. A great many simulation programs have been developed to study aspects of genetic change in other organisms, particularly Drosophila. Young (‘66, ’67), Hill (‘69), and Frankham et al.