Résumé
Understanding the genomic processes underlying local adaptation is a
central aim of modern evolutionary biology. This task requires identifying
footprints of local selection but also estimating spatio-temporal
variation in population demography and variation in recombination rate and
diversity along the genome. Here, we investigated these parameters in blue
tit populations inhabiting deciduous versus evergreen
forests and insular versus mainland areas, in the
context of a previously described strong phenotypic differentiation.
Neighboring population pairs of deciduous and evergreen habitats were
weakly genetically differentiated (FST = 0.004 on average),
nevertheless with a statistically significant effect of habitat type on
the overall genetic structure. This low differentiation was consistent
with the strong and long-lasting gene flow between populations,
inferred by demographic modeling. In turn, insular and mainland
populations were moderately differentiated (FST = 0.08 on
average), in line with the inference of moderate ancestral migrations,
followed by isolation since the end of the last glaciation. Effective
population sizes were overall large, yet smaller on the island than on the
mainland. Weak and non-parallel footprints of divergent selection between
deciduous and evergreen populations were consistent with their high
connectivity and the probable polygenic nature of local adaptation in
these habitats. In turn, stronger footprints of divergent selection were
identified between long isolated insular versus mainland
birds, and were more often found in regions of low recombination as
expected from theory. Lastly, we identified a genomic inversion on the
mainland, spanning 2.8Mb. These results provide insights into the
demographic history and genetic architecture of local adaptation in blue
tit populations at multiple geographic scales.