Population structure and anthropometric variation in rural western Ireland: migration and biological differentiation

Models of population structure can be investigated using data on anthropometric variation among local populations. Anthropometric data collected by Dupertuis and Dawson during the 1930s were analysed from 347 males and 261 females in 12 towns in three counties of western Ireland. We hypothesized that recent migration would decrease the degree of among-group variation. To test this hypothesis, two additional samples were created by excluding known inter-county migrants from both male and female samples. Based on ethnographic data, a fifth sample was created using unmarried females only, in order to control partially for local migration upon marriage. Univariate and multivariate measures of relative differentiation were developed to compare different levels of migration and differences among the sexes. We found that the degree of among-group variation decreased as the amount of migration increased, in accordance with spatial models of population structure. Using non-parametric correlations of geographic and anthropometric distance, the observed patterns of differentiation were closely related to geography, suggesting a spatial model of gene flow to be appropriate in interpreting among-group variation. The female samples showed greater differentiation and higher correlations with geography than the males. It seems that this results from the sensitivity of males to developmental and local environmental influences, causing an increase in the relative amount of within-group variation.