Ecological adaptation drives wood frog population divergence in life history traits

Phenotypic variation among populations is thought to be generated from
spatial heterogeneity in environments that exert selection pressures that
overcome the effects of gene flow and genetic drift. Here, we tested for
evidence of isolation by distance or by ecology (i.e., ecological
adaptation) to generate variation in early life history traits and
phenotypic plasticity among 13 wood frog populations spanning 1200 km and
7° latitude. We conducted a common garden experiment and related trait
variation to an ecological gradient derived from an ecological niche model
(ENM) validated to account for population density variation. Shorter
larval periods, smaller body weight and relative leg lengths were
exhibited by populations with colder mean annual temperatures, greater
precipitation, and less seasonality in precipitation, and higher
population density (high suitability ENM values). After accounting for
neutral genetic variation, the QST–FST analysis supported ecological
selection as the key process generating population divergence. Further,
the relationship between ecology and traits was dependent upon larval
density. Specifically, high suitability/high-density populations in the
northern part of the range were better at coping with greater conspecific
competition, evidenced by greater post-metamorphic survival and no
difference in body weight when reared under stressful conditions of high
larval density. Our results support that both climate and competition
selection pressures drive clinal variation in larval and metamorphic
traits in this species. Range-wide studies like this one are essential for
accurate predictions of population’s responses to ongoing ecological
change.