Abstract
Leapfrog patterns are a peculiar and little-understood phenomenon in which similar populations at either end of a geographic continuum are divided by dissimilar intervening populations. Leapfrog patterns may be important in allopatric speciation. Most documented cases of biological leapfrog patterns refer to morphological traits in passerine birds, and only few have been reported from outside the Andean region. More importantly, the biological basis of leapfrog patterns continues to elude biologists. We document a vocal leapfrog pattern—possibly the second such case documented—in the Maroon-chinned Fruit Dove (Ptilinopus subgularis) complex, adding to the few known examples of leapfrog patterns from outside the Neotropics and in non-passerines. P. subgularis is a Wallacean pigeon whose three taxa occur in a longitudinal continuum from Sulawesi in the west to the Sula Islands in the east. We used discriminant analysis and other statistical methods to demonstrate that terminal members of the complex differ in song from the geographically intermediate taxon but resemble each other more closely. Plumage in P. subgularis does not exhibit the same geographic distribution of variability, a pattern that agrees with the only other study reporting a vocal leapfrog pattern, and supports an earlier hypothesis that leapfrog patterns arise from stochastic phenotypic changes in geographically intermediate taxa. We combine the new insights on the distribution of vocal trait variation in P. subgularis with Pleistocene earth-historic information to revise species in the complex.