Spatial, temporal, and genome-wide patterns of differentiation for the Louisiana iris species complex
Hamlin, Jennafer Ashley Parker
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The classic mode of allopatric speciation emphasizes geographical isolation and assumes ecological factors as insignificant, whereas now there is renewed interest in highlighting the importance of ecology during speciation. Ecological speciation, as a potential by-product of local adaption, results when barriers to gene flow evolve between populations as an outcome of ecologically-based divergent natural selection. However, gene flow tends to counteract evolutionary forces such as natural selection because gene flow constrains population differentiation via genetic exchange, thereby preventing the evolution of complete reproductive isolation. Thus, work on young incipient species allows one to disentangle the influence of gene flow, ecological differentiation, and local adaptation during the early stages of divergence. The Louisiana irises are a species complex of relatively recent origin and here I focus on the three species that are morphologically distinct, widespread, and interfertile: Iris brevicaulis, Iris fulva, and Iris hexagona. I found that there are species-level differences in connectivity among populations of Louisiana irises; in that bumblebee pollinated species (I. brevicaulis and I. hexagona) are more restricted via intraspecific gene flow than I. fulva, which is pollinated by hummingbirds. Moreover, gene flow was detected in populations found in the southern parts of the range and occurred from I. fulva into I. brevicaulis. Furthermore, it appears that both selection and neutral processes are at play in generating population divergence for I. hexagona in terms of floral trait differences. This implies that the potential for divergent selection and local adaptation, in terms of pollinators, can promote diversification within a single lineage. Differences observed between species, with regard to associated environmental factors, suggest an effect from these components on the distributions and habitats occupied. Furthermore, niche divergence was implicated in all pairwise comparisons regardless of range overlap and niches remained differentiated after lineage diversification. Thus it appears that even in the face of gene flow, niche divergence can be maintained and that ecological speciation is the likely speciation mechanism for the Louisiana irises.