Abstract
Previous theory on adaptation to a changing environment has identified genetic variance as a key factor. Contemporary climate change renews the interest for such studies, which now attempt to include the complexity of life and climate change. Rapid evolution of flowering time has been documented in several species as a response to climate change. Assortative mating, i.e. mating restricted to individuals with similar phenotypes, is frequent and obligate for flowering time, the intensity of natural selection can differ between sexes, and the intensity of stabilizing selection on flowering can vary with the duration of seasons, and which fluctuates across years. These features are however largely ignored by extent theory on adaptation to changing environments. In a first chapter, we have studied the effects of assortative mating for flowering date on evolutionary responses to climate change, with the aim to evaluate whether assortative mating, compared to random mating, can explain fast evolutionary responses of flowering phenology to climate change. To this end, an individual-based quantitative genetics model simulates climate change and the evolution of mean individual flowering date in an isolated population. In most scenarios, and despite its negative effect on genetic polymorphism, assortative mating maintains higher genetic variance at equilibrium than random mating, and therefore allows populations to better track climate change and to have a better fitness. An analytic model, based on the infinitesimal model of trait heritability, confirms those results. The second chapter integrates more elements of realism, common in plant and animal populations: sex-specific natural selection and sexual dimorphism. The analytical model is an extension of the previous one, and generalizes results by including sexual dimorphism, and two frequently observed types of assortative mating in animals and plants: assortative preference of females for male phenotypes, and temporal assortative mating for flowering date. The model shows that (i) assortative mating generates sexual selection, which increases the effect of natural selection on females and decreases the effect of natural selection on males, (ii) when sexual dimorphism is large, assortative mating further generates directional sexual selection on male phenotypes, which can lead to the evolution of trait values overshooting the interval between the male and female optima; (iii) in some conditions, which occur both under random and assortative mating, female maladaptation can be smaller in a changing environment than in a constant environment; (iv) assortative mating can help populations to better track climate change than random mating, when selection on females is stronger than that on males and/or sexual dimorphism is not too large, and/or climate change is fast enough. The robustness of results has been tested with an individual-based model. The third chapter studies the effects of the fluctuations of the strength of selection on the long-term responses of populations. To this end, we have used both analytical approximations and a numerical exploration of the infinitesimal model. Fluctuations in the strength of selection are modeled by fluctuations of the width of the fitness function assumed to be Gaussian. Such fluctuations increase the mean strength of selection and therefore decrease genetic variance and adaptive lag. Fluctuations of the strength of selection however have a demographic cost, and decrease the long-run growth rate of populations in most cases. Taken together, these results suggest that: (i) assortative mating improves adaptation to climate change only under specific circumstances, (ii) rapid evolutionary responses to climate change do not necessarily mitigate its negative consequences on demography.