| Understanding the factors that contribute to the origin and maintenance of species, and elucidating the mechanisms that influence species' distribution across the landscape are two goals that are fundamental to evolutionary biology and ecology. I combined field and laboratory experiments, a robust phylogeny, and species distribution data from herbaria to test a series of hypotheses that address variation in the distribution of species, and, the evolution and maintenance of reproductive isolating barriers, key components for understanding speciation. I used species in the genus Collinisa to test the following main hypotheses: (1) An extension of Baker's Law: Among similar aged sister-taxa pairs, species more proficient at autonomous self-fertilization should be better colonizers and thus should have larger range sizes than their sister-taxa that are less proficient at autonomous selfing, (2) Bateson-Dobzhansky-Muller (BDM) model of reproductive isolation: Intrinsic postzygotic isolation barriers increase as divergence time increases among species, and (3) Reinforcement of reproductive isolation: early selfing can evolve in response to heterospecific pollen receipt, and may thus act as a prezygotic reproductive isolating barrier that is reinforced in sympatry. I found that species most proficient at selfing had significantly larger range sizes than their sister-taxa that were less proficient at selfing. Thus, mating system did explain differences in the ranges sizes of similar aged sister-taxa. To address the second hypothesis, I first tested for allopatric speciation in this genus, and found strong support for allopatric speciation in the California clade of Collinsia. But I did not find strong support overall, likely because of large range-shifts in the northeastern clade, which obscured the expected pattern of increasing range overlap with increasing divergence time. In support of the BDM model, I found that post-mating pre- and postzygotic isolation increased with increasing divergence time. And finally, in support of the final prediction, I found that when C. rattanii was sympatric with C. linearis it self-pollinated at a significantly earlier stage, suggesting that earlier selfing may be acting to reinforce of reproductive isolation in sympatry. |
