Résumé
Explaining the strong variation in lifespan among organisms remains a
major challenge in evolutionary biology. Whereas previous work has
concentrated mainly on differences in selection regimes and selection
pressures, we hypothesize that differences in genetic drift may explain
some of this variation. We develop a model to formalize this idea and show
that the strong positive relationship between lifespan and genetic
diversity predicted by this model indeed exists among populations of
Daphnia magna, and that ageing is accelerated in small populations.
Additional results suggest that this is due to increased drift in small
populations rather than adaptation to environments favoring faster life
histories: First, the correlation between genetic diversity and lifespan
remains significant after statistical correction for potential
environmental covariates. Second, no trade-offs are observed; rather, all
investigated traits show clear signs of increased genetic load in the
small populations. Third, hybrid vigor with respect to lifespan is
observed in crosses between small but not between large populations.
Together, these results suggest that the evolution of lifespan and ageing
can be strongly affected by genetic drift, especially in small
populations, and that variation in lifespan and ageing may often be
non-adaptive, due to a strong contribution from mutation accumulation to
this variation.