When can stress facilitate divergence by altering time to flowering?

Crispin Y. Jordan,Dilara Ally,Kathryn A. Hodgins

doi:10.1002/ece3.1821

Abstract

Stressors and heterogeneity are ubiquitous features of natural environments, and theory suggests that when environmental qualities alter flowering schedules through phenotypic plasticity, assortative mating can result that promotes evolutionary divergence. Therefore, it is important to determine whether common ecological stressors induce similar changes in flowering time. We review previous studies to determine whether two important stressors, water restriction and herbivory, induce consistent flowering time responses among species; for example, how often do water restriction and herbivory both delay flowering? We focus on the direction of change in flowering time, which affects the potential for divergence in heterogeneous environments. We also tested whether these stressors influenced time to flowering and nonphenology traits using Mimulus guttatus. The literature review suggests that water restriction has variable effects on flowering time, whereas herbivory delays flowering with exceptional consistency. In the Mimulus experiment, low water and herbivory advanced and delayed flowering, respectively. Overall, our results temper theoretical predictions for evolutionary divergence due to habitat-induced changes in flowering time; in particular, we discuss how accounting for variation in the direction of change in flowering time can either increase or decrease the potential for divergence. In addition, we caution against adaptive interpretations of stress-induced phenology shifts.

Highlights

Gene flow tends to homogenize populations, which reduces the potential for local adaptation (Lenormand 2002), speciation (Coyne and Orr 2004), and range expansion (Kirkpatrick and Barton 1997)
Herbivory showed a strong tendency to delay flowering, whereas low water tended to elicit earlier and delayed flowering with relatively equal frequency. If these same trends manifest within species, we expect that individual species will very frequently delay flowering in response to both low water and herbivory, but many others will shift their dates of first flowering in opposite directions in response to these stressors – these predictions apply most to species with similar qualities to those in our dataset, i.e., largely temperate species with annual life histories and a phylogenetic distribution similar to our dataset
For species that shift the date of first flowering in the same direction for both stressors, we predict that opportunities for evolutionary divergence via habitat-induced shift in date of first flowering (HISF) diminish in heterogeneous environments compared to environments with a single stressor, because the phenology shifts effectively cancel each other out

Summary

Introduction

Gene flow tends to homogenize populations, which reduces the potential for local adaptation (Lenormand 2002), speciation (Coyne and Orr 2004), and range expansion (Kirkpatrick and Barton 1997). Numerous forms of assortative mating can evolve that reduce gene flow and promote diversification (e.g., Doebeli and Dieckmann 2003), including mating within groups due to differential timing (Antonovics 2006) or location of reproduction (Otto et al 2008), and self-fertilization (e.g., Dickinson and Antonovics 1973; Epinat and Lenormand 2009). In this light, studying mechanisms that reduce gene flow is fundamental to understanding the maintenance and generation of biodiversity

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Conclusion