Evolution of multiple postzygotic barriers between species of the Mimulus tilingii complex

Species are often defined by their ability to interbreed (i.e., Biological
Species Concept), but determining how and why reproductive isolation
arises between new species can be challenging. In the Mimulus
tilingii species complex, three species (M.
caespitosa, M. minor, and M. tilingii) are largely
allopatric and grow exclusively at high elevations (>2000m). The
extent to which geographic separation has shaped patterns of divergence
among the species is not well understood. In this study, we determined
that the three species are morphologically and genetically distinct, yet
recently diverged. Additionally, we performed reciprocal crosses within
and between the species and identified several strong postzygotic
reproductive barriers, including hybrid seed inviability, F1 hybrid
necrosis, and F1 hybrid male and female sterility. In this study, such
postzygotic barriers are so strong that a cross between any species pair
in the M. tilingii complex would cause nearly complete
reproductive isolation. We consider how geographical and topographical
patterns may have facilitated the evolution of several postzygotic
barriers and contributed to speciation of closely related members within
the M. tilingii species complex.