The reproductive success in human populations depends on the total number of descendants that each couple (or each woman) leaves to subsequent generations. Natural selection does not work only through differential fertility, in fact biological fitness is also determined by the survival of the descendants, at least until their reproductive age is reached. The present study was conducted on the population of Chiomonte, a small rural village in the Susa Valley (Piedmont, Italy). We analyzed variations of the Crow’s index of opportunity for natural selection (It) and its components (Im and If) from 1670 to 1830. During this timeframe, major economic and social changes occurred in the community, which have had repercussions on certain aspects of the structure of the families and on their descendants. Drawing from marriage, baptism, and death acts recorded in the parish registers, it was possible to reconstruct "biological" families in order to analyse the reproductive history of about 80% of the married women, by assessing the number of children each of them had in relation to their age at marriage and its duration. The analysis of the interaction between mortality models and the opportunity for natural selection has allowed us to understand the potential implications of mortality crises and, above all, the role that infant mortality had in the past. This remains at values up to 250 ‰ until the second half of the eighteenth century and then reduces to mean values below 220 ‰, with a consequent reduction of the importance of the pre-reproductive mortality component on total selection. Two periods can be identified in the trend of It's variation: first, from 1670 until the middle of the eighteenth century, where it remains relatively constant, and a second in which it tends to decrease progressively. Furthermore, we observed a trend of progressive increase in the fertility component.
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