Thermal ecology and seasonal adaptation along climatic gradients
Seasonal timing, or phenology, determines how an organism's life history unfolds over the course of a year. When environments fluctuate seasonally, natural selection on a particular trait depends on position within a calendar year. For example, selection on anti-predator behavior and morphology in water fleas (Daphnia sp.) often depends on seasonal fluctuations in predator density in ephemeral ponds. A more universal example involves seasonal fluctuations in temperature. Organisms with complex life cycles may time active life history stages so that they occur only during the warmer periods of a temperate year, synchronizing dormant stages with harsh, winter conditions. The pitcher plant mosquito (Wyeomyia smithii), for example, overwinters as a dormant (diapausing) larvae in the water-filled leaves of the purple pitcher plant, Sarracenia purpurea. These larvae cue the initiation and termination of diapause on day length. Larvae metamorphose from pupa to adult during the spring and early summer, and larval diapause is initiated in late summer or fall.
As ectotherms, insect growth, performance, and reproduction are highly temperature-dependent, making environmental temperature an important agent of natural selection. Active growth and reproduction takes place primarily during the summer, and any selection imposed during active life history stages depends on temperatures experienced only between diapause termination and initiation. Thus, changing the timing of diapause initiation or termination changes temperature-mediated selection on non-dormant life history stages.
We generally expect that higher latitude or altitude populations of a broadly distributed species will be more cold adapted because average temperatures decline with increasing latitude and altitude. Likewise, we expect the opposite trend for heat adaptation. Timing of diapause often evolves along geographic clines, however, affecting expected trends in selection. For example, evolved differences in diapause timing cause geographically disparate populations of pitcher plant mosquitoes to experience fairly comparable low temperature exposure during times when they are actively growing and reproducing.
I am interested in how this interaction between seasonal timing and natural selection affects the evolution of thermal adaptation. I have primarily addressed this question through comparative, common garden experiments on thermal plasticity in geographic populations of the pitcher plant mosquito, focusing on active life history stages. I am also exploring the simultaneous effects of seasonal timing and annual/diurnal temperature variation on fitness via meta-analyses of latitudinally-distributed insect species and empirically-informed optimality models. Preliminary results suggest that 1) greater latitudinal predictability of the timing of seasonal transitions compared to annual average or extreme temperatures may explain observed latitudinal patterns of thermal physiology and dormancy timing, and 2) although fitness surfaces may be rugged, seasonal timing and thermal parameters may have equivalently large effects on fitness (i.e., depending on heritability, selection may be equivalently strong on each type of trait).
Currently I am addressing related questions in apple maggot flies centering on adaptation in the diapause stage itself. Eventually I hope to integrate these studies to obtain a more complete picture of thermal ecology and adaptation across the entire life cycle.