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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 Mange’s inbreeding coefficients and Lasker and Kaplan’s repeated pair values were calculated using 8026 marriage records; temporal changes were assessed by dividing birth cohorts into 4 time periods of 25 years each: 1887-1911,1912-1936,1937-1961, and 1962-1986. Analysis was carried out at 2 hierarchical levels: the population of the valley as a whole and the valley’s subdivision into 21 parishes. The relationship between population size and level of isonymy during the breakdown of isolates was investigated. The results show that there is a small difference in inbreeding coefficients between the first 2 periods at either hierarchical level of analysis and a substantial decrease in isonymy in the last period. Analysis by sex showed that the decrease in marital isonymy during the study period is mostly due to the change in male random isonymy. Furthermore, the Fn value at the higher hierarchical level almost coincides with the mean F value at the lower hierarchical level, indicating that over time the parish remained the fundamental reproductive unit. Regression analysis showed that geographic isolation became increasingly important in differentiation among the parishes in population size and in levels of inbreeding. The marked deviation from equilibrium between drift and migration that characterizes the breakdown of isolates of almost all the rural populations is an important disturbing factor in assessing the relationship between level of inbreeding and population size. Comparison over time allows us to better describe the evolutionary forces at the basis of the changes in genetic structure of a population.