The evolution of sweet chestnut (Castanea sativa Mill.) appears to be a complex mixture of long-range gene flow, natural and artificial selection, and local effects of isolation by distance. In this paper we present the most complete analysis to date on the genetic structure and variability of 52 populations of chestnut spanning the entire European area of distribution. The study is based on the use of isozyme data. Our samples came from four major zones, possibly representing relevant steps in the evolution and spread of sweet chestnut in Europe: (i) eastern Turkey, the supposed center of origin of the species; (ii) western Turkey, the area in which human domestication started; (iii) Italy, where domesticated chestnut was first introduced to the rest of Europe by the Romans; and (iv) France, representing the latest phases of the expansion, close to the northern limit of the taxon.
As previous studies based only on Italian and some Turkish populations suggested, the electrophoretic data are consistent with a series of episodes of west- and north-ward migration. The early expansion from the center of origin was probably slow, resulting from natural diffusion of the species. Most of the original genetic variation has been conserved during this phase. Successive episodes of colonization of western Turkey and then of the rest of Europe were probably the more rapid result of human activity. These later stages were associated with genetic drift that reduced the overall heterozygosity of the extant populations. No evidence for selection could be found at the large geographical scale of this study, although previous regional works have shown spatial patterns of allelic frequencies at a few loci and phenotypic differentiation consistent with the action of past selective pressures.