COS 159-1 - CANCELLED - Life history tradeoffs in persistence and speed of adaptation in changing environments

Background/Question/Methods

Life history strategies with iteroparity spread out risk of reproductive failure over multiple mating bouts, reducing extinction risk by stabilizing population growth rates in randomly varying environments. However, iteroparous life history strategies with longer lifespans have slower generational turnover and potentially keep maladapted genotypes in the mating pool for longer, slowing rates of adaptation to directional environmental change. This produces a trade-off, where iteroparity buffers populations from potential extinction but slows evolution. This trade-off has been noted before, but it is often attributed to iteroparity’s association with complex life cycles that allow populations to “store” individuals in stages subject to the least selection pressure. We hypothesize that this trade-off would also exist in a population with a single stage class. We test this hypothesis with simulation models where life history is determined solely by survival and reproduction for one stage class. We simulate three strategies: “fast” (low survival, high annual fecundity), “medium”, and “slow” (high survival, low annual fecundity) with selection on survival. We simulated populations for 100 timesteps in environments under a large, single change and gradual change while varying the magnitude of random environmental fluctuations, recording population size, extinction, and mean phenotype in each time step.

Results/Conclusions

Even in populations without stage structure, we observed a trade-off between rates of persistence and speed of adaptation to environmental change. For populations tracking a single environmental shift, “slow” life history populations had slower population decline but also had the slowest rate of adaptation. Consequently, slow life histories took longer to recover positive growth rates than faster life histories. These patterns were similar under gradual environmental shifts: slow populations maintained the highest mean population growth rate but had the largest phenotypic lag behind the shifting environment. When conditioning mean phenotype on age (age 1 recruit versus age > 1 surviving adult), the fast life histories had less phenotypic lag than medium and fast life history strategies in both new recruits and surviving adults. Increasing the magnitude of environmental fluctuations increased extinction rates in all treatment groups, but had the largest effect on fast life histories, in part because faster life histories with faster adaptation responded more to large, anomalous fluctuations. These results confirm the existence of a trade-off between population stability and speed of adaptation but demonstrate that longer-lived individuals contribute to this process by contributing increasingly maladapted genotypes to the mating pool as populations attempt to track directional change.