Loss of a morph is associated with asymmetric character release in a radiation of woodland salamanders

Color polymorphism, the occurrence of discrete color morphs with co-adapted sets of traits within the same population, may provide the raw materials for rapid species formation. The fixation of a morph can result in character release, whereby the monomorphic form evolves without the constraint of accommodating multiple adaptive peaks. However, rates of evolution between populations fixed for different morphs likely depend on their specific adaptive zones. We studied dorsal color polymorphism (striped and unstriped morphs) in woodland salamanders (Plethodon), a North American radiation in which the polymorphism can be found in even the most distantly related species (∼44 Ma divergence). We estimated a phylogenomic tree, representing all extant taxa and including multiple samples for most species. Morphometric data suggest species vary along an axis of body elongation, corresponding to a terrestrial-fossorial continuum. Polymorphic species occupy an intermediate phenotypic optimum between striped and unstriped species. Faster rates of body shape evolution were observed in only small-bodied unstriped species, suggesting that body elongation, which is co-adapted with the unstriped morph, could be constrained by polymorphism. Our results demonstrate that rates of phenotypic evolution and speciation following character release can be asymmetric and idiosyncratic depending on the alternative adaptations of each morph.