In numerous clades, divergent sister species have largely non-overlapping geographic ranges. This pattern presumably arises because species diverged in allopatry or parapatry, prior to subsequent contact. Here we provide population-genomic evidence for the opposite scenario: previously sympatric ecotypes that have spatially separated into divergent monomorphic populations over large geographic scales (reverse sympatric scenario). We analyzed a North American salamander (Plethodon cinereus) with two color morphs that are broadly sympatric: striped (redback) and unstriped (leadback). Sympatric morphs can show considerable divergence in other traits, and many Plethodon species are fixed for a single morph. Long Island (New York) is unusual in having many pure redback and leadback populations that are spatially separated, with pure redback populations in the west and pure leadbacks in the east. Previous work showed that these pure-morph populations were genetically, morphologically, and ecologically divergent. Here, we performed a coalescent-based analysis of new data from 88,696 single-nucleotide polymorphisms to address the origins of these populations. This analysis strongly supports the monophyly of Long Island populations and their subsequent divergence into pure redback and pure leadback populations. Taken together, these results suggest that the formerly sympatric mainland morphs separated into parapatric populations on Long Island, reversing the conventional speciation scenario.

Files for Dryad:

File S1. Zipped directory containing R commands and datasets for generating figures and analyses.

File S2. Input file with data and analysis settings (XML format) for SNAPP analysis of Plethodon cinereus from Long Island and adjacent mainland populations using all SNPs.

File S3. Input file with data and analysis settings (XML format) for SNAPP analysis of Plethodon cinereus from Long Island and adjacent mainland populations using first-SNP only.

*Raw data from Illumunia Hi-Seq will be uploaded to the NCBI sequence read archive (SRA) 

R statistical environment

BEAST 2.6.3