Résumé
Hybridization is increasingly recognized as a significant evolutionary
process, in particular because it can lead to introgression of genes from
one species to another. A striking pattern of discordance in the amount of
introgression between mitochondrial and nuclear markers exists such that
substantial mitochondrial introgression is often found in combination with
no or little nuclear introgression. Multiple mechanisms have been proposed
to explain this discordance, including positive selection for
introgressing mitochondrial variants, several types of sex-biases, drift,
negative selection against introgression in the nuclear genome, and
spatial expansion. Most of these hypotheses are verbal, and have not been
quantitatively evaluated so far. We use individual-based, multi-locus,
computer simulations of secondary contact under a wide range of
demographic and genetic scenarios to evaluate the ability of the different
mechanisms to produce discordant introgression. Sex-biases and spatial
expansions fail to produce substantial mito-nuclear discordance. Drift and
nuclear selection can produce strong discordance, but only under a limited
range of conditions. In contrast, selection on the mitochondrial genome
produces strong discordance, particularly when dispersal rates are low.
However, commonly used statistical tests have little power to detect this
selection. Altogether, these results dismiss several popular hypotheses,
and provide support for adaptive mitochondrial introgression.