Data from: Phenotypic plasticity for sailfin mollies raised at two temperatures and three salinities

Phenotypic plasticity, the ability of a single genotype to produce
different phenotypes under different environmental conditions, plays a
profound role in several areas of evolutionary biology.  However,
it is unclear how much reaction norms vary among conspecific populations
and whether differences in reaction norms represent adaptations to
different levels of environmental variation.  We present data on
the reaction norms of growth and maturation to variation in temperature
and salinity in sailfin mollies (Poecilia latipinna) from three
populations from South Carolina (SC).  This area is the northern
edge of the species range and we compare these reaction norms to those
previously reported in populations from north Florida (NF), which is the
center of the species range.  In general, fish from SC displayed
less plasticity than fish from NF because they grew faster and matured
earlier at the lower temperatures and salinities compared to NF
fish.  Among fish from both regions, males were much less plastic
than females.  In both sexes, fish from SC matured at much
younger ages than NF fish in the same conditions.  While there
was no detectable heterogeneity among populations from NF, males from one
of the SC populations displayed a strikingly different response in age at
maturity to temperature variation than did males from the other two SC
populations, maturing earlier at the lower temperature, rather than at the
higher one.  The pattern in fish from SC is one of
countergradient variation, in which they grow faster and mature earlier in
conditions that would otherwise slow growth and delay maturity. 
This pattern is well-matched to the cooler thermal regime and shorter
growing season experienced in SC populations, suggesting an adaptive
divergence in reaction norms.