Differential Adaptation to a Harsh Granite Outcrop Habitat between Sympatric Mimulus Species

A primary goal in evolutionary biology is to understand which environmental variables and traits drive adaptation to harsh environments. This is difficult since 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 M. 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 x M. guttatus F4 hybrids. We find that differences in drought & 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. We conclude that while 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.