Abstract
Deterministic seasonality can explain the evolution of alternative life history phenotypes (i.e., life history polyphenism) expressed in different generations emerging within the same year. However, the influence of stochastic variation on the expression of such life history polyphenisms in seasonal environments is insufficiently understood. Here, we use insects as a model and explore (1) the effects of stochastic variation in seasonality and (2) the life cycle on the degree of life history differentiation among the alternative developmental pathways of direct development and diapause (overwintering), and (3) the evolution of phenology. With numerical simulation, we determine the values of development (growth) time, growth rate, body size, reproductive effort, adult life span, and fecundity in both the overwintering and directly developing generations that maximize geometric mean fitness. The results suggest that natural selection favors the expression of alternative life histories in the alternative developmental pathways even when there is stochastic variation in seasonality, but that trait differentiation is affected by the developmental stage that overwinters. Increasing environmental unpredictability induced a switch to a bet‐hedging type of life history strategy, which is consistent with general life history theory. Bet‐hedging appeared in our study system as reduced expression of the direct development phenotype, with associated changes in life history phenotypes, because the fitness value of direct development is highly variable in uncertain environments. Our main result is that seasonality itself is a key factor promoting the evolution of seasonally polyphenic life histories but that environmental stochasticity may modulate the expression of life history phenotypes.
Highlights
The effects of temporal environmental uncertainty on life history evolution have been extensively studied
Divergent life history phenotypes were generally predicted to evolve in the diapause and direct development pathways in the analyzed selection regimes whenever bivoltine phenology evolved (Figs. 3, 4)
Increasing interyear standard deviation of season length induced a switch from bivoltine to univoltine phenology (Figs. 3, 4), indicating that environmental variation constrains the evolution of bivoltinism
Summary
The effects of temporal environmental uncertainty on life history evolution have been extensively studied. Life history theory predicts that temporal uncertainty of the environment favors temporal dispersion of reproduction (i.e., iteroparity; Schaffer 1974a; Tuljapurkar and Wiener 2000; Wilbur and Rudolf 2006; but see Orzack and Tuljapurkar 1989) and/or variable or delayed time of maturation (Cohen 1966; Tuljapurkar 1990; Tuljapurkar and Istock 1993; Menu et al 2000; Tuljapurkar and Wiener 2000; Wilbur and Rudolf 2006; Koons et al 2008). Risk spreading by a single genotype expressing different phenotypes reduces fitness variance and/or between-individual correlations in fitness and is known as evolutionary “bet-hedging” (Seger and Brockmann 1987; Phillippi and Seger 1989; Starrfelt and Kokko 2012). Diversified bet-hedging strategies (sensu Starrfelt and Kokko 2012) arise via environmental effects so that alternative phenotypes are expressed with certain probabilities to environmental cues, a phenomenon known as adaptive “coin flipping” (Cooper and Kaplan 1982) and “stochastic polyphenism” (Walker 1986)
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