Biomedical subjects

John H Relethford

Publications and source records attributed to John H Relethford.

5 recordsLinked to original sources

PubMed2004

Global patterns of isolation by distance based on genetic and morphological data.

The isolation-by-distance model predicts that genetic similarity between populations will decrease exponentially as the geographic distance between them increases, because of the limiting effect of geographic distance on rates of gene flow. Many studies of human populations have applied the isolation-by-distance model to genetic variation between local populations in a limited geographic area, but few have done so on a global level, and these few used different models and analytical methods. I assess genetic variation between human populations across the world using data on red blood cell polymorphisms, microsatellite DNA markers, and craniometric traits. The isolation-by-distance model provides an excellent fit to average levels of genetic similarity within geographic distance classes for all three data sets, and the rate of distance decay is the same in all three. These results suggest that a common pattern of global gene flow mediated by geographic distance is detectable in diverse genetic and morphological data. An alternative explanation is that the correspondence between genetic similarity and geographic distance reflects the history of dispersal of the human species out of Africa.

Biological Evolution↗

PubMed2004

Local extinction and recolonization, species effective population size, and modern human origins.

A primary objection from a population genetics perspective to a multiregional model of modern human origins is that the model posits a large census size, whereas genetic data suggest a small effective population size. The relationship between census size and effective size is complex, but arguments based on an island model of migration show that if the effective population size reflects the number of breeding individuals and the effects of population subdivision, then an effective population size of 10,000 is inconsistent with the census size of 500,000 to 1,000,000 that has been suggested by archeological evidence. However, these models have ignored the effects of population extinction and recolonization, which increase the expected variance among demes and reduce the inbreeding effective population size. Using models developed for population extinction and recolonization, we show that a large census size consistent with the multiregional model can be reconciled with an effective population size of 10,000, but genetic variation among demes must be high, reflecting low interdeme migration rates and a colonization process that involves a small number of colonists or kin-structured colonization. Ethnographic and archeological evidence is insufficient to determine whether such demographic conditions existed among Pleistocene human populations, and further work needs to be done. More realistic models that incorporate isolation by distance and heterogeneity in extinction rates and effective deme sizes also need to be developed. However, if true, a process of population extinction and recolonization has interesting implications for human demographic history.

Animals↗

PubMed2002

Apportionment of global human genetic diversity based on craniometrics and skin color.

A number of analyses of classical genetic markers and DNA polymorphisms have shown that the majority of human genetic diversity exists within local populations (approximately 85%), with much less among local populations (approximately 5%) or between major geographic regions or "races" (approximately 10%). Previous analysis of craniometric variation (Relethford [1994] Am J Phys Anthropol 95:53-62) found that between 11-14% of global diversity exists among geographic regions, with the remaining diversity existing within regions. The methods used in this earlier paper are extended to a hierarchical partitioning of genetic diversity in quantitative traits, allowing for assessment of diversity among regions, among local populations within regions, and within local populations. These methods are applied to global data on craniometric variation (57 traits) and skin color. Multivariate analysis of craniometric variation shows results similar to those obtained from genetic markers and DNA polymorphisms: roughly 13% of the total diversity is among regions, 6% among local populations within regions, and 81% within local populations. This distribution is concordant with neutral genetic markers. Skin color shows the opposite pattern, with 88% of total variation among regions, 3% among local populations within regions, and 9% within local populations, a pattern shaped by natural selection. The apportionment of genetic diversity in skin color is atypical, and cannot be used for purposes of classification. If racial groups are based on skin color, it appears unlikely that other genetic and quantitative traits will show the same patterns of variation.

Cephalometry↗

PubMedDate not supplied

Heritability and anthropometric influences on human fertility.

This study researched the impact of anthropometrics and size-of-family of orientation on women's fertility by using path analysis. The data were collected as part of the anthropological study conducted in Ireland by Harvard University personnel before the Second World War. The women included in this analysis were all over age 49 and were either married or widowed at the time of the survey. Our results indicate that the heritability of fertility is moderate in this sample and that there is a tendency for heavy women to have a higher fertility. However, when anthropometrics and size-of-family of orientation were entered as independent variables in a path diagram, an insignificant portion of the variation of fertility was explained. In this Irish population, the main cause of differential fertility was cultural rather than biological. A large portion of women never married and no unmarried woman reported producing a child.

Anthropometry↗

PubMedDate not supplied

Boas and beyond: migration and craniometric variation.

Migration is expected to affect craniometric variation in three ways: 1) movement into a different environment leading to developmental plasticity; 2) movement into a different environment followed by in situ adaptation through natural selection; and 3) changes in among-group differentiation and genetic distance through the action of gene flow. The relative influence of these three factors has been argued in the literature, most recently in a series of articles debating the statistical and biological significance of Boas's immigration studies as they relate to cranial plasticity. The Boas debate is discussed within the broader context of debate over genetic and environmental influences on craniometric variation. Additional examples are provided from an ongoing study of global craniometric variation. Although developmental plasticity and climatic adaptation have had an impact on craniometric variation, these factors tend not to erase, or even obscure greatly, underlying patterns of population structure and history that fit a neutral model of quantitative variation. Thus, craniometric data can be used to explore questions of gene flow and genetic affinity.

Adult↗