Résumé
In heterogeneous landscapes, the genetic and demographic consequences of
dispersal influence the evolution of niche width. Unless pollen is
limiting, pollen dispersal does not contribute directly to population
growth. However, by disrupting local adaptation, it indirectly affects
population dynamics. We compare the effect of pollen versus seed dispersal
on the evolution of niche width in heterogeneous habitats, explicitly
considering the feedback between maladaptation and demography. We consider
two scenarios: the secondary contact of two subpopulations, in distinct,
formerly isolated habitats, and the colonization of an empty habitat with
dispersal between the new and ancestral habitat. With an analytical model,
we identify critical levels of genetic variance leading to niche
contraction (secondary contact scenario), or expansion (new habitat
scenario). We confront these predictions with simulations where the
genetic variance freely evolves. Niche contraction occurs when habitats
are very different. It is faster as total gene flow increases or as pollen
predominates in overall gene flow. Niche expansion occurs when habitat
heterogeneity is not too high. Seed dispersal accelerates it, whereas
pollen dispersal tends to retard it. In both scenarios very high seed
dispersal leads to extinction. Overall, our results predict a wider niche
for species dispersing seeds more than pollen.