Changes over 100 years in degree of isolation of 21 parishes of the Lima Valley, Italy, assessed by surname isonymy

Changes over 100 years (1887-1986) in degree of isolation of 21 parishes of the Lima valley, Italy, were assessed using surname analysis. Crow and Man ge's inbreeding coefficients and Lasker and Kaplan's repeated pair values w ere calculated using 8026 marriage records; temporal changes were assessed by dividing birth cohorts in 4 time periods of 25 years each: 1887-1911, 19 12-1936, 1937-1961, and 1962-1986. Analysis was carried out at 2 hierarchic al levels: the population of the valley as a whole and the valley's subdivi sion into 21 parishes. The relationship between population size and level o f isonymy during the breakdown of isolates was investigated. The results sh ow that there is a small difference in breeding coefficients between the fi rst 2 periods at either hierarchical level of analysis and a substantial de crease in isonymy in the last period. Analysis by sex showed that the decre ase in marital isonymy during the study period is mostly due to the change in male random isonymy. Furthermore, the F-n value at the higher hierarchic al level almost coincides with the mean F value at the lower hierarchical l evel, indicating that over time the parish remained the fundamental reprodu ctive unit. Regression analysis showed that geographic isolation became inc reasingly important in differentiation among the parishes in population siz e and in levels of inbreeding. The marked deviation from equilibrium betwee n drift and migration that characterizes the breakdown of isolates of almos t all the rural populations is an important disturbing factor in assessing the relationship between level of inbreeding and population size. Compariso n over time allows us to better describe the evolutionary forces at the bas is of the changes in genetic structure of a population.