Understanding Evolutionary Impacts of Seasonality: An Introduction to the Symposium

Abstract

Seasonality is a critically important aspect of environmental variability, and strongly shapes all aspects of life for organisms living in highly seasonal environments. Seasonality has played a key role in generating biodiversity, and has driven the evolution of extreme physiological adaptations and behaviors such as migration and hibernation. Fluctuating selection pressures on survival and fecundity between summer and winter provide a complex selective landscape, which can be met by a combination of three outcomes of adaptive evolution: genetic polymorphism, phenotypic plasticity, and bet-hedging. Here, we have identified four important research questions with the goal of advancing our understanding of evolutionary impacts of seasonality. First, we ask how characteristics of environments and species will determine which adaptive response occurs. Relevant characteristics include costs and limits of plasticity, predictability, and reliability of cues, and grain of environmental variation relative to generation time. A second important question is how phenological shifts will amplify or ameliorate selection on physiological hardiness. Shifts in phenology can preserve the thermal niche despite shifts in climate, but may fail to completely conserve the niche or may even expose life stages to conditions that cause mortality. Considering distinct environmental sensitivities of life history stages will be key to refining models that forecast susceptibility to climate change. Third, we must identify critical physiological phenotypes that underlie seasonal adaptation and work toward understanding the genetic architectures of these responses. These architectures are key for predicting evolutionary responses. Pleiotropic genes that regulate multiple responses to changing seasons may facilitate coordination among functionally related traits, or conversely may constrain the expression of optimal phenotypes. Finally, we must advance our understanding of how changes in seasonal fluctuations are impacting ecological interaction networks. We should move beyond simple dyadic interactions, such as predator prey dynamics, and understand how these interactions scale up to affect ecological interaction networks. As global climate change alters many aspects of seasonal variability, including extreme events and changes in mean conditions, organisms must respond appropriately or go extinct. The outcome of adaptation to seasonality will determine responses to climate change.

The genetic architecture underlying diapause termination in a planktonic crustacean

“…Diapause stages are often resistant to environmental stressors like cold, heat or drought, but on the other hand cannot exploit beneficial habitat conditions for growth or reproduction. The exact timing of diapause induction and termination are thus fitness relevant life‐history traits through which these organisms integrate in the overall phenology of the habitat (Forrest & Miller‐Rushing, ; Williams et al, ). Phenology, the timing of biological events, here refers to the seasonally elevated availability of resources like nutrients or food items, but also of predators, parasites or detrimental abiotic factors (e.g., winter cold), whose fluctuations over the course of the year are recurrent and predictable (Williams et al, ).…”

“…The exact timing of diapause induction and termination are thus fitness relevant life‐history traits through which these organisms integrate in the overall phenology of the habitat (Forrest & Miller‐Rushing, ; Williams et al, ). Phenology, the timing of biological events, here refers to the seasonally elevated availability of resources like nutrients or food items, but also of predators, parasites or detrimental abiotic factors (e.g., winter cold), whose fluctuations over the course of the year are recurrent and predictable (Williams et al, ). Diapause timing is achieved through the interpretation of environmental cues, which can be abiotic cues like photoperiod or temperature, or biotic cues like predator kairomones or a reduction in food availability or quality (Walsh, ).…”

“…Our capability to predict the evolution of life history phenotypes requires, however, information on its physiological underpinnings and the genetic architecture controlling these traits, as the number of genes and their effect, genetic linkage and epistatic interactions can impose evolutionary constraints or facilitate adaptation (Flatt & Heyland, ). This knowledge gap has been emphasized specifically for the seasonal timing of diapause (Williams et al, ), which mostly occurs in response to photoperiod (Bradshaw & Holzapfel, ). For invertebrate diapause, most progress has been made in identifying genes underlying local adaptation in diapause induction (reviewed in Meuti & Denlinger, ), which were repeatedly found to be associated with the circadian clock (Denlinger, Hahn, Merlin, Holzapfel, & Bradshaw, ).…”

Phenotypic plasticity, but not genetic adaptation, underlies seasonal variation in the cold hardening response ofDrosophila melanogaster

“…For example, aspects of cold tolerance are known to vary as a function of seasonal exposure and provide a mechanism for some species to successfully overwinter (Esterbauer and Grill 1978; Anderson et al 1992; Shearer et al 2016). While phenotypic variation can arise as a result of environmentally triggered plasticity, genetic variation in seasonally advantageous traits also exists (Dobzhansky and Ayala 1973; reviewed in Tauber and Tauber 1981 and Williams et al 2017). Therefore, genotypes that underlie variation in seasonally relevant phenotypes may change in frequency across seasonal timescales for short-lived organisms (King 1972; Grosberg 1988; Hazel 2002; Schmidt and Conde 2006; Behrman et al 2015).…”

Cold adaptation does not alter ATP homeostasis during cold exposure in Drosophila melanogaster

“…However, what constitutes an "extreme" differs markedly between organisms, making it challenging to link projected changes in climate to organismal responses. An understanding of the functional mechanisms underlying responses to climate extremes is essential to predicting impacts of climate change on animal distributions, and can aid in identifying precise conditions under which populations will be most challenged under predicted climate changes (Pörtner 2001;Huey et al 2012;Williams et al 2017). Functional understanding is equally important to identifying the generality or lack thereof of specific mechanisms underlying loss of organismal function (Somero 2010).…”