The effects of demographic transition on the opportunity for selection: changes during the last century in Italy

The index of opportunity for selection proposed by Crow has been calculated for the Italian population during the last century. The evolution of its two components, the pre-reproductive mortality and the variance in fertility, has been also analysed and compared with similar data for the United States. The results clearly show the relevance of socio-economic changes to the evolution of selective patterns in our species; in particular the relative incidence of mortality and fertility to selection intensity: the total index has in fact been reduced by 75% during the last century, the relative amount due to fertility increasing from 57 to 89%. The probable different relevance of genetic factors in the two components has also been discussed.