Genetic drift and gene flow in post-famine Ireland.

This study examines the genetic impact of the Great Famine (1846-1851) on the regional genetic structure of Ireland. The Great Famine resulted in a rapid decrease in population size throughout Ireland in a short period of time, increasing the possibility of genetic drift. Our study is based on migration and anthropometric data collected originally in the 1930s from 7211 adult Irish males. These data were subdivided into three time periods defined by year of birth: 1861-1880, 1881-1900, and 1901-1920. Within each time period the data were further subdivided into six geographic regions of Ireland. Estimates of Wright's FST were calculated from parent-offspring migration data and from 17 anthropometric variables (10 head measures, 7 body measures). Over time, the average population size decreased, but average rates of migration increased. The estimates of FST at equilibrium from migration matrix analysis suggest that the net effect of these opposite effects is a reduction in among-group variation. Closer examination shows that within each time period the rate of convergence to equilibrium is slow, meaning that the expected levels of genetic homogeneity revealed from migration matrix analysis are not likely to be seen over short intervals of time. Estimates of FST from anthropometric data show either relatively little change in microdifferentiation or some increase, depending on which variables are analyzed. Investigation of a simple model of demographic and genetic change shows that, given the demographic changes in post-Famine Ireland, FST could in theory increase, decrease, or remain the same over short intervals of time. Overall, the Great Famine appears to have had minimal impact on the genetic structure of Ireland on a regional level. Comparison with studies focusing on local genetic structure shows the opposite. It appears that the level of genetic impact depends strongly on the level of analysis; local populations are affected to a greater extent by demographic shifts than regional populations. We also provide formulas for the standard errors of FST from metric traits and related statistics.