Differential Adaptation to a Harsh Granite Outcrop Habitat between SympatricMimulusSpecies

Abstract

Understanding which environmental variables and traits underlie adaptation to harsh environments is difficult because many traits evolve simultaneously as populations or species diverge. Here, we investigate the ecological variables and traits that underlie Mimulus laciniatus’ adaptation to granite outcrops compared to its sympatric, mesic‐adapted progenitor, Mimulus guttatus. We use fine‐scale measurements of soil moisture and herbivory to examine differences in selective forces between the species’ habitats, and measure selection on flowering time, flower size, plant height, and leaf shape in a reciprocal transplant using M. laciniatus × M. guttatus F4 hybrids. We find that differences in drought and herbivory drive survival differences between habitats, that M. laciniatus and M. guttatus are each better adapted to their native habitat, and differential habitat selection on flowering time, plant stature, and leaf shape. Although early flowering time, small stature, and lobed leaf shape underlie plant fitness in M. laciniatus’ seasonally dry environment, increased plant size is advantageous in a competitive mesic environment replete with herbivores like M. guttatus’. Given that we observed divergent selection between habitats in the direction of species differences, we conclude that adaptation to different microhabitats is an important component of reproductive isolation in this sympatric species pair.

Divergence in drought‐response traits between sympatric species of Mimulus

“…Thus, timing of the treatments in the two studies was similar rela- (Ivey & Carr, 2012;Wu et al, 2010) and under natural conditions in the field (Ferris & Willis, 2018;Hall & Willis, 2006;Mojica et al, 2012).…”

Adaptation and divergence in edaphic specialists and generalists: serpentine soil endemics in the California flora occur in barer serpentine habitats with lower soil calcium levels than serpentine tolerators

“…It may be that adaptation to drought, or any of these other selective pressures, contributes to trade‐offs in competitive ability instead of adaptation to serpentine soil chemistry. For example, shifts to earlier flowering times are common in serpentine plants (Rajakaruna, ; Wright, Davies, et al., ; Kay et al., ; Dittmar and Schemske, ) and are hypothesized to evolve as a mechanism to escape drought in rocky serpentine habitats (Brady et al., ; Ferris and Willis, ), but earlier flowering may come with a trade‐off in growth that would be disadvantageous in a competitive environment. A quantitative trait locus (QTL) mapping study between Microseris douglasii (DC.)…”

Strong stabilizing selection on timing of germination in a Mediterranean population of Arabidopsis thaliana

“…This is unfortunate because such variation in optimal germination time would be important to consider for an understanding of (a) the relative performance of different genotypes in natural populations and (b) the relative importance of differences in environmental conditions and in genetic composition for among‐population variation in phenotypic selection on germination timing. Information about optimal timing of germination could also be important when interpreting the results of common‐garden and reciprocal transplant experiments where germination of different genotypes is typically synchronized and seedlings planted on a given day (e.g., Antonovics and Primack, 1982; Schmid, 1985; Wang and Redmann, 1996; Fournier‐Level et al, 2011; Ågren et al, 2013; Ferris and Willis, 2018; Wadgymar et al, 2018; Lowry et al, 2019). If optimal germination time varies among genotypes, planting date should affect their relative fitness.…”