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Abstract
During range expansions, even low levels of interbreeding can lead to massive introgression
of local alleles into an invader's genome. Nonetheless, this pattern is not always
observed in human populations. For instance, European Americans in North America are
barely introgressed by Amerindian genes in spite of known contact and admixture. With
coalescent spatially explicit simulations, we examined the impact of long-distance
dispersal (LDD) events on introgression of local alleles into the invading population
using a set of different demographic scenarios applicable to a diverse range of natural
populations and species. More specifically, we consider two distinct LDD models: one
where LDD events originate in the range core and targets only the expansion front
and a second one where LDD events can occur from any area to any other. We find that
LDD generally prevents introgression, but that LDD events specifically targeting the
expansion front are most efficient in suppressing introgression. This is likely due
to the fact that LDD allows for the presence of a larger number of invader alleles
at the wave front, where effective population size is thus increased and local introgressed
alleles are rapidly outnumbered. We postulate that the documented settlement of pioneers
directly on the wave front in North America has contributed to low levels of Amerindian
admixture observed in European Americans and that this phenomenon may well explain
the lack of introgression after a range expansion in natural populations without the
need to evoke other mechanisms such as natural selection.