Population genomic analyses support sympatric origins of parapatric morphs in a salamander

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.