The evolution of optimal emergence times: bet hedging and the quest for an ideal free temporal distribution of individuals

Abstract

Proper timing of activities is one of the principal challenges faced by most organisms. Organisms need to account for various aspects in decision making like avoiding inordinate risks, synchronizing with resource availability, or finding mates. We provide analytical and simulation models to investigate the influence of life expectancy, resource competition and unpredictable environmental conditions (environmental uncertainty) on the evolutionarily stable distribution of emergence times in organisms depending on seasonally available resources. We focus on the partitioning of total phenotypic variance in emergence times into 1) genetic variance in mean emergence times between lineages and 2) environmental trait variance that determines the intra-lineage variance in the timing of emergence.Both, life expectancy of organisms and intensity of competition severely influence the evolutionary response to environmental uncertainty. Our main findings can be summarized as follows: 1) in general diversifying bet hedging (environmental trait variance) is the adequate mechanism to reduce the risk arising from environmental uncertainty while conservative bet hedging, i.e. delaying emergence into 'safe' phases of the season is restricted to short lived organisms and to situations with vanishing competition. 2) Environmental trait variance increases with increasing environmental uncertainty whereas 3) significant genetic variance evolves only under severe resource competition; it is driven by selection for an ideal free distribution of emergence times. 4) The level of genetic variance evolving declines with increasing life expectancy of organisms. 5) With sufficiently short life expectancy evolutionary branching and coexistence of distinctly different emergence strategies occurs; the number of co-occurring strategies is determined by the level of environmental uncertainty.Our model provides cues for understanding how different ecological factors contribute and interact to shape the evolution of emergence strategies.The proper timing of life-history events like pupation or emergence in insects, is an issue of far reaching fitness consequences (Bradshaw and Holzapfel 2007). Timing is important to meet periods of favorable abiotic and biotic environmental conditions and avoid unfavorable situations, e.g. by initiating a diapause (Schmidt et al. 2005). Emergence (or eclosion), for example, must be timed to avoid periods of adverse conditions like frost or drought. Ill-adapted timing may indeed limit a (geno)type's distribution (Chuine 2010). For example, in a controlled experiment mosquitoes of the species Wyeomyia smithii lost 74% (southern populations) and even 88% (northern population) of their fitness compared to the best-timed intermediate population (Bradshaw et al. 2004). Similar reasoning may apply to the proper timing of daily activities like foraging (Emerson et al. 2008, Miller-Rushing et al. 2010. Observed emergence patterns can be as complex as e.g. the trimodal strategy of the rose sawfly (Kawasaki et al. 2012).Timing i...

Optimal germination timing in unpredictable environments: the importance of dormancy for both among‐ and within‐season variation

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Spatial and temporal heterogeneity in climate change limits species' dispersal capabilities and adaptive potential

“…However, plasticity is unlikely to be beneficial in situations in which the cues used by an individual to induce plastic changes are no longer predictive of future environmental conditions (McNamara et al 2016). Such scenarios should instead lead to the development of bet-hedging strategies, but these may be slow to evolve in all but the most dramatically unpredictable and resource-limited environments (Poethke et al 2016). Similarly, only the introduction of novel genetic variation may be able to adaptively resolve situations in which two antagonistic traits are experiencing directional selection in opposing directions (Careau et al 2015), suggesting that evolutionary responses to the contrasting selection pressures imposed by asynchronous regimes may be especially slow.…”

Overwintering temperature and body condition shift emergence dates of spring-emerging solitary bees

“…As the season progresses the number of emerged bees increases, and also if a large number of bees emerge simultaneously, intra-specific competition for mating partners (in the case of males) and inter- and intra-specific competition for nesting sites (in the case of females) are increasing. Individuals emerging earlier may thus gain fitness benefits ( Poethke, Hovestadt & Mitesser, 2016 ). Choosing the right moment for emergence may therefore require balancing these different risks and benefits.…”

“…However, solitary bees show considerable variability in emergence dates even if they overwinter at the same location with the same overwintering temperature ( Westrich, 2011 ). To our knowledge, the mechanisms and causes underlying this variation cannot be explained with certainty, but the speculation is a (maternal) bet-hedging strategy, which can be expected to pay off in environments with unpredictable environmental variability ( Danforth, 1999 ; Hopper, 1999 ; Childs, Metcalf & Rees, 2010 ; Poethke, Hovestadt & Mitesser, 2016 ). Our study tries to clarify the ultimate and proximate causes of this high variability in emergence dates of solitary bees.…”