Life histories and phenologies are assumed to evolve in accordance with natural environmental fluctuations such as seasonal cycles. Currently across the discipline of ecology, there are many disparate hunts for system-specific correlates of phenological patterns, especially in light of climate change and phenological shifts. However, even though phenology is a ubiquitous phenomenon, ultimate evolutionary drivers underlying how life histories are adapted to environmental cycles remain surprisingly poorly understood. System-specific correlations may yield limited lessons for our general understanding of selective pressures underlying phenology and life history. In my previous work I developed a species-agnostic but flexible demographic framework that predicts how cycles should shape whole life history strategies, and how strategies should change as cyclicity changes. I tuned this model and successfully tested it across natural populations of an intertidal copepod Tigriopus californicus that span a wide gradient of tide cycle periods. In the present study, I take these natural populations into the laboratory, and conduct a long-term evolutionary experiment using varying cycle period treatments as well as stochastic treatments for ~6 generations. This experiment tests the strength of the demographic mechanisms assumed in the theory for inducing life history evolution.
Results/Conclusions
Over only ~6 generations, laboratory populations of T. californicus sourced from different natural populations shifted in their life histories (maturation rate and fecundity) under varying cyclical treatments. The shifts were achieved purely from cyclical manipulations to demographic dynamics, without the possibility of plastic responses to environmental cues which is the common proximate layer of causality explored in phenology studies. Different cycle periods induced different directions of life history evolution, predicted by the model. Stochastic treatments induced increased variance in life histories within experimental populations but not directional change. These results confirm the power of the central assumptions of the model that was designed to be applicable to any species of interest. Importantly, this experiment suggests an explanation for why life histories and phenologies are shifting in different directions for different species in nature—causing phenological mismatches among interacting species—when a system’s cyclicity is perturbed by climate change.