Effects of population structure on within-group variation in the Jirels of Nepal

The impact of population structure on phenotypic differentiation is most frequently considered in terms of between-subdivision variation. However, the demographic and social structures of a population also induce changes in within-group variation. We analyze the intragenerational dynamics of within-group variation for the Jirels, a tribal population of eastern Nepal. In the analyses we utilized age- and sex-corrected cranial measures (head length, head breadth, bizygomatic diameter, minimum frontal diameter, and head circumference) available for 526 adults (ages 15-54 years). We used a multivariate measure of variance, the standardized generalized variance, to assess levels of within-village variability, quantifying the sampling variance for this statistic by using a jackknife methodology. To generate null expectations of within-group variation, we used permutation procedures, which permit robust testing of significance without distributional assumptions. We also compared the within-birthplace variation in adults to the observed within-residence variation to examine migration effects. Contrary to expectation, some villages with high rates of in-migration have less variability than those with few migrants. When differences between the sexes at birth are controlled for, females in some villages exhibit greater variance than males, reflecting known differences in sex-specific dispersal.