Contrasting phylogeographic histories between broadly sympatric topminnows in the Fundulus notatus species complex.

Sympatrically distributed closely related species provide opportunities for studying evolutionary patterns of diversification. Such studies must account for historical contingencies in interpreting contemporary patterns of variation. Topminnows in the Fundulus notatus species complex are distributed sympatrically across much of the southern and Midwestern United States. Throughout most of their ranges F. olivaceus is often found in headwater stream habitats, and F. notatus is more typically distributed along the margins of larger river habitats. However, in some drainages, ecological associations of the respective species are reversed, with F. notatus populations isolated in headwater streams and F. olivaceus in downstream river habitats. Phylogeographic analyses of AFLP marker and multi-locus sequence data detected historical isolation in F. notatus consistent with pre-Pleistocene drainage patterns. Four F. notatus clades corresponded to (i) the Western Gulf Slope, (ii) the southwestern Ouachita Highlands, (iii) the Mobile Basin, and (iv) central Coastal Plain and Mississippi River Basin. In contrast, a relative lack of range-wide geographic structure in F. olivaceus is consistent with recent range expansion over much of the same geographic area. The southwestern Ouachita Highlands and Mobile Basin F. notatus clades corresponded to regions where ecological associations between the two species are reversed, providing evidence of the independent evolution of variation in contemporary habitat associations. Fundulus olivaceus from several drainages demonstrated introgression of mitochondrial DNA from F. notatus, but none of the sites in this study included individuals with hybrid ancestry in their nuclear genome. Phylogenetic analyses that included only nuclear loci supported the reciprocal monophyly of F. notatus, F. olivaceus and a third narrowly endemic species, Fundulus euryzonus, and supported a sister relationship between F. olivaceus and F. euryzonus.