Phenotypic plasticity and local adaptation in tadpoles

Amphibian larvae are plastic in many traits. Can plasticity buffer population divergence, so that local adaptation is more common in non-plastic traits? I argue that this may be the case for adaptive plastic responses. Tadpoles of Rana arvalis are shown to alter their tail depth plastically as a response to predation risk; this is a truly adaptive response. Tail depths in the field (population and year mean values) fall between the phenotypes produced by raising tadpoles in the lab with and without predator cues. There is no clear evidence for population differentiation (i.e., genetically) in tail depth, as would be expected if the tadpoles were not able to adjust their phenotypes to the environmental condition by adaptive plasticity.

On the contrary, should the plastic response not be a truly adaptive response but merely the result of a constraint, evolution may counter the plastic effect by pushing the trait in the opposite direction. Such a phenomenon is suggested for developmental rate: while predator presence causes tadpoles to develop slower (a non-adaptive consequence of an adaptive activity reduction), tadpoles from ponds that are "risky" had a higher developmental rate than tadpoles from comparatively "safe" ponds when raised under identical conditions. A study on allozyme variation across the study area suggests that the gene flow between sites is low enough to allow divergence in selected characters, but sufficient to prevent genetic drift. I show that taxa differ in what traits are plastically altered as an anti-predator response, and attempt to correlate overall plasticity to habitat variability. Data suggest that the variability within a population in the relative allocation to different defence strategies is small.

Tadpoles of R. arvalis from sites where they occur in allopatry (on island Gotland) and in sympatry with Rana temporaria were raised in presence and absence of the congeneric. The fundamental food niche of the allopatric population seems not to have been altered after isolation on Gotland - interspecific competitive effects were as strong in both populations. However, the Gotlandian tadpoles tended to be more sensitive to competition in general.

Tadpoles of frogs breeding in seaside pools were not more tolerant to saline conditions than those from inland sites. This may be due to a rarity of selection events combined with gene flow from inland sites.

In sum, population divergence depends not only on gene flow and the strength of selection, but also on phenotypic plasticity and whether the plastic responses are adaptive or not. Since many amphibian populations are declining, translocations have been proposed and used as a tool in conservation. I discuss the pros and cons of this, with reference to plasticity and local adaptation.