Life history data on married women of the historical population of île aux Coudres

Evidence from natural populations shows that changes in environmental
conditions can cause rapid modifications in the evolutionary potential of
phenotypes, partly through genotype-by-environment interactions (G×E).
Therefore, the overall rate of microevolution should depend on
fluctuations in environmental conditions, even when directional selection
is sustained over several generations. We tested this hypothesis in a
preindustrial human population that experienced a microevolutionary change
in age at first reproduction (AFR) of mothers, using the annual infant
mortality rate (IMR) as an indicator of environmental conditions during
their early life. Using quantitative genetics analyses, we found that G×Es
explained a non-negligible fraction of the additive genetic variance in
AFR and in relative fitness, as well as of the genetic covariance between
AFR and fitness (i.e. the Robertson-Price covariance). The covariance was
stronger for individuals exposed to unfavorable early-life environmental
conditions. Our results unravel the presence of G×Es in an important life
history trait and its impact on the rate of microevolution, which appears
to have been sensitive to short-term fluctuations in local environmental
conditions.