Data from: Population size, habitat fragmentation, and the nature of adaptive variation in a stream fish

Whether and how habitat fragmentation and population size jointly affect
adaptive genetic variation and adaptive population differentiation are
largely unexplored. Owing to pronounced genetic drift, small, fragmented
populations are thought to exhibit reduced adaptive genetic variation
relative to large populations. Yet fragmentation is known to increase
variability within and among habitats as population size decreases. Such
variability might instead favour the maintenance of adaptive polymorphisms
and/or generate more variability in adaptive differentiation at smaller
population size. We investigated these alternative hypotheses by analysing
coding-gene, single-nucleotide polymorphisms associated with different
biological functions in fragmented brook trout populations of variable
sizes. Putative adaptive differentiation was greater between small and
large populations or among small populations than among large populations.
These trends were stronger for genetic population size measures than
demographic ones and were present despite pronounced drift in small
populations. Our results suggest that fragmentation affects natural
selection and that the changes elicited in the adaptive genetic
composition and differentiation of fragmented populations vary with
population size. By generating more variable evolutionary responses, the
alteration of selective pressures during habitat fragmentation may affect
future population persistence independently of, and perhaps long before,
the effects of demographic and genetic stochasticity are manifest.