Speciation is one of the great mysteries of evolution. As organisms adapt to their environment, they could diverge in traits that are important for reproductive isolation. When taxa become reproductively isolated, speciation is considered complete. Here, I approach the speciation process from a broad perspective, using genus level phylogenetics, and a narrow perspective, focusing on the mechanisms of isolation between two species.
Genus level phylogenetics reveal patterns in adaptive traits, traits correlated with diversification, and form the basis for hypotheses about adaptive evolution. First, I present the first comprehensive phylogeny of diploid Dudleya (Crassulaceae) species, a California radiation that is highly variable. I resolved four clades within Dudleya, found many variable groups to be polyphyletic, and supported the monophyly of the genus. Unique soil affinities, such as serpentine tolerance, appear to have evolved numerous times. Many relationships are still uncertain, however, and sequence variation is lacking at the markers I used. The lack of sequence variation could be due to recent evolution of the genus or reoccurring hybridization between taxa. The molecular phylogeny presented here will form the foundation for future work on the diversification of the genus.
In chapters two and three I focus on the specific mechanisms that isolate two close relatives in the genus Lasthenia (Asteraceae). I document different patterns of adaptation at two sites where the species co-occur through reciprocal transplant experiments. I then use simulation models to link adaptation to other mechanisms of reproductive isolation, including flowering time, post-pollination isolation, and postzygotic isolation. I provide evidence that barrier strength changes depending on the ecological context. Some traits can function as barriers to gene flow in some ecological contexts, but promote gene flow in other contexts. I found that although total isolation was similar between species and sites, the barriers that contributed to that isolation were variable in identity and magnitude. This work expands our understanding of how ecology can contribute to speciation. link to publication