Most current models of human population structure view migration solely as a deterministic force reducing the variance in gene frequencies among the local colonies of a subdivided population. By an empirical example and through simulation experiments, it is shown that migration structured along kinship lines (by analogy to the lineal or 'kinship' effect) does not always reduce the variances of gene frequencies arising through intergenerational random genetic drift. Thus populations experiencing high rates of migration may not be genetically homogenous.
A Monte Carlo simulation based on the population structure of a small-scale human population, the Semai Senoi of Malaysia, has been developed to study the combined effects of group, kin, and individual selection. The population structure resembles D.S. Wilson's structured deme model in that local breeding populations (Semai settlements) are subdivided into trait groups (hamlets) that may be kin-structured and are not themselves demes. Additionally, settlement breeding populations are connected by two-dimensional stepping-stone migration approaching 30% per generation. Group and kin-structured group selection occur among hamlets the survivors of which then disperse to breed within the settlement population. Genetic drift is modeled by the process of hamlet formation; individual selection as a deterministic process, and stepping-stone migration as either random or kin-structured migrant groups. The mechanism for group selection is epidemics of infectious disease that can wipe out small hamlets particularly if most adults become sick and social life collapses. Genetic resistance to a disease is an individual attribute; however, hamlet groups with several resistant adults are less likely to disintegrate and experience high social mortality. A specific human gene, hemoglobin E, which confers resistance to malaria, is studied as an example of the process. The results of the simulations show that high genetic variance among hamlet groups may be generated by moderate degrees of kin-structuring. This strong microdifferentiation provides the potential for group selection. The effect of group selection in this case is rapid increase in gene frequencies among the total set of populations. In fact, group selection in concert with individual selection produced a faster rate of gene frequency increase among a set of 25 populations than the rate within a single unstructured population subject to deterministic individual selection. Such rapid evolution with plausible rates of extinction, individual selection, and migration and a population structure realistic in its general form, has implications for specific human polymorphisms such as hemoglobin variants and for the more general problem of the tempo of evolution as well.
Analysis of histories and genealogies from seven relatively unacculturated, swidden-farming Semai settlements shows that the composition of local groups fluctuates through time. This instability is similar to a pattern which Neel and his colleagues have suggested is typical of primitive society, the fission-fusion model. In addition, the individuals comprising Semai fission groups are kinsmen which implies that the number of independent genomes represented is markedly less than the number of individual migrants (the lineal effect). Fission groups may form new villages or fuse with an established settlement. In either case, the genetic effects of such migration are more pronounced than would be expected on the basis of founder effect or random migration. Despite several conspicuous differences in social organization between the Semai and the South American Indians (e.g., bilateral vs. unilineal descent) whose population structure provided the empirical basis for the fission-fusion, lineal effect model, the basic similarities are striking. The Semai case thus lends support to the proposition that this pattern may be of some generality in technologically primitive populations.
An excess of male over female deaths is characteristic of modern national populations, whereas in some high-mortality societies female mortality exceeds that of males. Among the Semai Senoi, a Malaysian Orang Asli ("aboriginal") population, women experienced higher mortality than males in the decades before 1969. This differential occurred in all age classes older than 15 years so that the sex ratio progressively increased with age. A recent (1987) restudy of the Semai population found that sex-specific differential mortality is much reduced. A comparison of the 1969 and 1987 life tables shows a sharp shift in the sex ratios of mortality for the post-15-year-old age classes (the geometric means of age classes 15-44 were 0.768 in 1969 and 0.997 in 1987) so that male and female expectations of further life at age 15 are now nearly identical. In contrast to the best-known cases of high female mortality (mostly in South Asia), Semai sex differential mortality does not include the childhood ages. The Semai have traditionally been relatively sexually egalitarian, and sex bias in care has not occurred. Analysis of sex-specific causes of death for the pre-1969 population suggests that maternal mortality is the major cause of the excess female deaths. The reduced number of maternal deaths seems largely due to better health care, particularly the availability of hospital services. Interestingly, the reduction in female mortality has occurred simultaneously with increased fertility, and overall mortality has continued at relatively high levels (eO less than 36). Thus, rather than forming a component of a unitary demographic transition, declining sex differences in mortality can be accounted for by a specific factor, better maternal care.
Blood samples, demographic and cultural data were collected from seven settlements of Semai Senoi, a swidden farming ethnic group of Malaysia. Three genetic loci (ABO blood group, hereditary ovalcytosis, and hemoglobin) were analyzed in a total sample of 546 individuals. These data indicate a considerable degree of genetic microdifferentiation in this area of the Semai distribution. Parent-offspring birthplace data (analyzed by means of a migration matrix) and settlement histories show that settlements are not strongly isolated. Genetic differences in the study area demonstrate a reasonable correspondence with migration and the history of the settlements. Genetic convergence also occurs through the addition of migrant groups to established populations leading to new patterns of marriage between donor and recipient groups. The genetic structure of the total Semai population through time thus comprises a mosaic of shifiting allele frequencies in a series of semi-isolated local populations.
Hereditary ovalocytosis/elliptocytosis occurs in polymorphic frequencies among several Malaysian populations and also in Melanesia. Although the condition has been described as an autosomal dominant, Melanesian family studies suggest that it is inherited recessively. Based on 75 Orang Asli families, it is shown that the Malaysian form of elliptocytosis is most likely inherited as an autosomal dominant. It appears, therefore, that either the inference of recessive inheritance in Melanesians is incorrect or that the ovalocytosis/elliptocytosis phenotypes are due to distinct genetic entities in the two regions.