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As part of an interdisciplinary research program on Alpine populations, we studied the biodemographic evolution of two populations of the Dauphiné in the period 1690–1799.We analyzed several indexes derived from surname analysis to infer the genetic structure of the populations. Although situated in the same area of the Dauphiné, the two communities of Vallouise and Chiomonte had different biodemographic characteristics. Vallouise was heavily populated but genetically homogeneous, whereas Chiomonte was less populated but more heterogeneous. The two districts also differed in geographic position: Vallouise was a glacier-enclosed valley that did not attract new inhabitants; Chiomonte was situated in an open valley served by important roads and thuswas able to attract many newinhabitants. The demographic differences between the two populations explain the differences in genetic structure. The index of isonymous relationship (Ri) being different from 0 is due to the rare marriages between members of the two populations. Because Ri is based on surnames, which are mostly polyphyletic, it can overestimate the genetic relationships between the populations, as in the case of consanguinity assessed by matrimonial isonymy.