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

Color polymorphism, the occurrence of multiple discrete color morphs with
co-adapted sets of traits within the same population, may provide the raw
materials for rapid species formation. It has been hypothesized that
fixation of a single 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 the specific adaptive zones occupied by
each morph. We studied the evolution of 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 of Plethodon, representing all extant taxa and including multiple
samples across the range of most species. Morphometric data suggest that
between-species variation exists predominantly along an axis of relative
body elongation, corresponding to a terrestrial–fossorial continuum.
Polymorphic species occupy an intermediate phenotypic space between the
evolutionary optima of 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 the polymorphism. Striped species, but also
large-bodied species of eastern Plethodon that lack a dorsal stripe, had
slower rates of phenotypic evolution. 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.