Population differentiation in Pacific salmon: local adaptation, genetic drift, or the environment?

Morphological, behavioral, and life-history differences between
Pacific salmon (Oncorhynchus spp.) populations are commonly thought to
reflect local adaptation, and it is likewise common to assume that salmon
populations separated by small distances are locally adapted. Two
alternatives to local adaptation exist: random genetic differentiation
owing to genetic drift and founder events, and genetic homogeneity among
populations, in which differences reflect differential trait expression in
differing environments. Population genetics theory and simulations suggest
that both alternatives are possible. With selectively neutral alleles,
genetic drift can result in random differentiation despite many strays per
generation. Even weak selection can prevent genetic drift in stable
populations; however, founder effects can result in random differentiation
despite selective pressures. Overlapping generations reduce the potential
for random differentiation. Genetic homogeneity can occur despite
differences in selective regimes when straying rates are high. In sum,
localized differences in selection should not always result in local
adaptation. Local adaptation is favored when population sizes are large and
stable, selection is consistent over large areas, selective diffeentials
are large, and straying rates are neither too high nor too low.
Consideration of alternatives to local adaptation would improve both
biological research and salmon conservation efforts.