Résumé
At species’ range edges, individuals often face novel environmental
conditions that may limit range expansion until populations adapt. The
potential to adapt depends on genetic variation upon which selection can
act. However, populations at species’ range edges are often genetically
depauperated. One mechanism to increase genetic variation is to reshuffle
existing variation through sex. During range expansions, sex can, however,
act as a double-edged sword. The gene swamping hypothesis predicts that
for populations expanding along an abiotic gradient, sex can hinder
adaptation if asymmetric dispersal leads to numerous maladapted dispersers
from the range core swamping the range edge. In this study, we
experimentally tested the gene swamping hypothesis by performing
replicated range expansions in landscapes with or without an abiotic
pH-gradient, using the ciliate Tetrahymena thermophila, while
simultaneously manipulating the occurrence of gene flow and sex. We show
that sex accelerated evolution of local adaptation in the absence of gene
flow, but hindered it in the presence of gene flow. The effect of sex,
however, was independent of the pH-gradient, indicating that not only
abiotic gradients but also the biotic gradient in population density leads
to gene swamping. Overall, our results show that gene swamping can affect
adaptation in life-history strategies.