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Ecogeographical and intrinsic postzygotic isolation between MIMULUS GLAUCESCENS and MIMULUS GUTTATUS
The mechanisms by which species are formed is a central question in evolutionary biology. The Biological Species Concept has provided a framework for quantifying, via reproductive isolation, the extent of divergence between populations. The relative strengths and importance of individual reproductive isolation barriers is unclear. This study estimated the strength of ecogeographic (prezygotic) isolation and intrinsic (postzygotic) isolation barriers (genetic incompatibilities between hybrids) between the closely related annual monkeyflowers, Mimulus glaucescens and M. guttatus. Georeferenced herbarium records and environmental data were used to model each species ecological niche and to measure their habitat overlap. Reciprocal crosses between species were performed to measure hybrid seed set, germination success of the resulting seeds, biomass of F1 adult plants, total number of days to first flower (developmental rate), total flower production, ovule numbers, and pollen viability, as intrinsic postzygotic isolation. The species exhibited both ecogeographical isolation and non-reciprocal intrinsic isolation (observed in seed set, developmental rate, and ovule production). These results suggest that isolation was a result of geologic history causing divergent ecological selection, where ecogeographical isolation acted before and limited a minimum of 45% gene flow compared to intrinsic postzygotic isolation factors, which acted later and limited a minimum of 21% of the gene flow on average between M. glaucescens and M. guttatus. The morphological differences between these species may reflect their genetic differences due to local adaptation.