Selection on phenotypic plasticity favors thermal canalization

Climate change affects organisms worldwide with profound ecological and
evolutionary consequences, often increasing population extinction risk.
Climatic factors can increase the strength, variability or direction of
natural selection on phenotypic traits, potentially driving adaptive
evolution. Phenotypic plasticity in relation to temperature can allow
organisms to maintain fitness in response to increasing temperatures,
thereby “buying time” for subsequent genetic adaptation and promoting
evolutionary rescue. Although many studies have shown that organisms
respond plastically to increasing temperatures, it is unclear if such
thermal plasticity is adaptive. Moreover, we know little about how natural
and sexual selection operate on thermal reaction norms reflecting such
plasticity. Here, we investigate how natural and sexual selection shape
phenotypic plasticity in two congeneric and phenotypically similar
sympatric insect species. We show that the thermal optima for longevity
and mating success differ, suggesting temperature-dependent trade-offs
between survival and reproduction in both sexes. Males in these
species have similar thermal reaction norm slopes but have diverged in
baseline body temperature (intercepts), being higher for the more northern
species. Natural selection favoured reduced thermal reaction norm slopes
at high ambient temperatures, suggesting that the current level of thermal
plasticity is maladaptive in the context of anthropogenic climate change
and that selection now promotes thermal canalization and robustness.
 Our results show that ectothermic animals also at high latitudes
can suffer from overheating and challenge the common view of phenotypic
plasticity as being beneficial in harsh and novel environments.