Résumé
Reduced population size is thought to have strong consequences for
evolutionary processes as it enhances the strength of genetic drift. In
its interaction with selection, this is predicted to increase the genetic
load, reduce inbreeding depression, and increase hybrid vigour, and in
turn affect phenotypic evolution. Several of these predictions have been
tested, but comprehensive studies controlling for confounding factors are
scarce. Here we show that populations of Daphnia magna, which vary
strongly in genetic diversity, also differ in genetic load, inbreeding
depression, and hybrid vigor in a way that strongly supports theoretical
predictions: Inbreeding depression is positively correlated with genetic
diversity (a proxy for Ne), and genetic load and hybrid vigour are
negatively correlated with genetic diversity. These patterns remain
significant after accounting for potential confounding factors and
indicate that, in small populations, a large proportion of the segregation
load is converted into fixed load. Overall, the results suggest that the
nature of genetic variation for fitness-related traits differs strongly
between large and small populations. This has large consequences for
evolutionary processes in natural populations, such as selection on
dispersal, breeding systems, ageing, and local adaptation.