Ecological divergence in flowering plants: niche expansion, selection, and reproductive isolation
The origin and persistence of species is fundamental to the evolution of biological diversity. Species generally arise in new environments through a combination of ecological divergence and spatial isolation. Under this framework, speciation occurs through three main processes: 1) niche expansion, 2) divergent selection, and 3) the accumulation of reproductive isolating mechanisms. In this dissertation, I tested specific hypotheses about the factors governing each of these stages, using a combination of stochastic simulations and field and greenhouse experiments with life history ecotypes of common monkeyflower Mimulus guttatus DC (Phrymaceae). First, I tested the ecological and genetic determinants of colonization success in flowering plants. In particular, I used stochastic simulations to test the effect of mating system plasticity on the persistence and adaptation of colonizing populations in new environments. I found that a shift to a mixed mating strategy facilitated niche expansion by reducing extinction and promoting adaptation under a broad range of colonization scenarios. In my second and third chapters, I examined the causes and consequences of life history divergence among populations of M. guttatus in central California. In chapter two, I used comparative demography to test for local adaptation and selection on life history traits in a common garden experiment in a montane perennial environment. I found that a perennial life history strategy had the highest performance and that selection favored perennial life history traits, although the native montane perennial ecotype was not locally adapted relative to other inland perennial populations. In chapter three, I quantified multiple isolating mechanisms between a focal montane perennial population and ten other populations spanning the range of life history strategies. I found that total reproductive isolation increased linearly with life history divergence between populations. However, total isolation was achieved through a mosaic of reproductive barriers and enhancers which, individually, exhibited non-linear relationships with life history divergence. These non-linear relationships are likely due to the effects of life history and inbreeding on the evolution of specific isolating mechanisms. Taken together, this work highlights the potential for diverse evolutionary, ecological, and demographic forces to interact to determine progress during the early stages of divergence.