GENETIC DRIFT AND GENE FLOW IN POSTFAMINE IRELAND

This study examines the genetic impact of the Great Famine (1846-1851) on the regional genetic structure of Ireland. The Great Famine result ed in a rapid decrease in population size throughout Ireland in a shor t period of time, increasing the possibility of genetic drift. Our stu dy is based on migration and anthropometric data collected originally in the 1930s from 7211 adult Irish males. These data were subdivided i nto three time periods defined by year of birth: 1861-1880, 1881-1900, and 1901-1920. Within each time period the data were further subdivid ed into six geographic regions of Ireland. Estimates of Wright's F-ST were calculated from parent-offspring migration data and from 17 anthr apometric variables (10 head measures, 7 body measures). Over time, th e average population size decreased, but average rates of migation inc reased. The estimates of F-ST at equilibrium from migration matrix ana lysis suggest that the net effect of these opposite effects is a reduc tion in among-group variation. Closer examination shows that within ea ch time period the rate of convergence to equilibrium is slow, meaning that the expected levels of genetic homogeneity revealed from migrati on matrix analysis are not likely to be seen over short intervals of t ime. Estimates of F-ST 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 demog raphic and genetic change shows that, given the demographic changes in post-Famine Ireland, F-ST could in theory increase, decrease, or rema in the same over short intervals of time. Overall, the Great Famine ap pears to have had minimal impact on the genetic structure of Ireland o n a regional level. Comparison with studies focusing on local genetic structure shows the opposite. It appears that the level of genetic imp act depends strongly on the level of analysis; local populations are a ffected to a greater extent by demographic shifts than regional popula tions. We also provide formulas for the standard errors of F (ST) from metric traits and related statistics.