PHENOTYPIC PLASTICITY IN THE SAILFIN MOLLY III. GEOGRAPHIC VARIATION IN
REACTION NORMS OF GROWTH AND MATURATION TO TEMPERATURE AND SALINITY
Abstract
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.