A heuristic model on the role of plasticity in adaptive evolution : 6 plasticity increases adaptation , population viability , and genetic variation

A heuristic model on the role of plasticity in adaptive evolution: 6 SUMMARY 28 2 An ongoing new synthesis in evolutionary theory is expanding our view of the sources 29 of heritable variation beyond point mutations of fixed phenotypic effects to include 30 environmentally-sensitive changes in gene regulation. This expansion of the paradigm is 31 necessary given ample evidence for a heritable ability to alter gene expression in 32 response to environmental cues. In consequence, single genotypes are often capable of 33 adaptively expressing different phenotypes in different environments, i.e. are adaptively 34 plastic. We present an individual-based heuristic model to compare the adaptive 35 dynamics of populations composed of plastic or non-plastic genotypes under a wide 36 range of scenarios where we modify environmental variation, mutation rate, and costs of 37 plasticity. The model shows that adaptive plasticity contributes to the maintenance of 38 genetic variation within populations, reduces bottlenecks when facing rapid 39 environmental changes, and confers an overall faster rate of adaptation. In fluctuating 40 environments, plasticity is favoured by selection and maintained in the population. 41 However, if the environment stabilises and costs of plasticity are high, plasticity is 42 reduced by selection, leading to genetic assimilation, which could result in species 43 diversification. More broadly, our model shows that adaptive plasticity is a common 44 consequence of selection under environmental heterogeneity, and hence a potentially 45 common phenomenon in nature. Thus, taking adaptive plasticity into account 46 substantially extends our view of adaptive evolution. 47 48