View Program - 2022 Population, Evolutionary, and Quantitative Genetics Conference

When environmental change is unpredictable and rapid, risk reduction becomes vital to the long term success for a lineage. Bet hedging is one possible strategy for risk reduction in varying environments. Here, a lineage sacrifices its short term fitness in order to insulate itself against future risk. Bet hedging is defined as any strategy that lowers temporal fitness variance at the expense of mean fitness; strategies are classified as either conservative or diversified. Examples include bacterial persistence, within-clutch egg size variation in salmon, and delayed seed germination in annual plants. Previous theory has relied on deterministic models, and has found that bet hedging is adaptive only when the geometric mean fitness of the bet hedger is greater than that of the specialist wild-type. While a few models have explicitly incorporated stochasticity in the environmental regime, the role of demographic stochasticity, or genetic drift, has been entirely overlooked in models of bet hedger evolution. We utilize a novel stochastic framework to simultaneously capture the effects of stochasticity in the environment and in reproductive output. Consistent with deterministic expectation, we find that when the geometric mean fitness of the bet hedger is greater than that of the resident wild-type specialist, the bet hedger is positively selected. However, we find that even if the geometric mean fitness of the bet hedger is modestly less than that of the resident wild-type, bet hedging can still be beneficial. More specifically, bet hedging is favored at large population sizes but disfavored at small population sizes. This phenomenon, where the sign of selection is dependent on population size, is known as sign inversion. Sign inversion has been previously observed in modifiers of mutation rate, recombination, and others, but this research is the first finding that bet hedging also exhibits sign inversion. Both conservative and diversified bet hedging strategies exhibit sign inversion in these regimes. Bet hedging only exhibits sign inversion when both stochasticity in reproduction and environment are considered; the essential interaction between these sources of stochasticity drives the probability of fixation above the neutral expectation. Our results demonstrate that bet hedging strategies can be adaptive in a larger range of environmental conditions than previously thought.

Don’t put all your eggs in one basket: what stochastic modeling tells us about bet hedger evolution