Ultrafast evolutionary morphological change in the Mallorca Midwife toad (Alytes muletensis, Alytidae)

Abstract

The Mallorcan Midwife toad (Alytes muletensis) is presently an endemic species living in some relict mountain areas of the Balearic Island of Mallorca. The known fossil record from Pleistocene and Holocene deposits indicates that most probably the species was widespread throughout the island before the arrival of human populations. The osteological comparison between the living relict populations, restricted to very particular biotopes, and their Quaternary fossil ancestors offer a unique opportunity to study short time evolutionary changes along an anuran chrono-species. This study uses standard and geometric morphometrics to analyze intraspecific variation in size and shape of the radioulna of fossil (a late Pleistocene or early Holocene sample from Cova de la Barxa) and Recent (non bred in captivity) specimens. Multivariate regression, principal components, canonical variates analyses as well as MANOVA indicate highly significant differences in radioulnar morphology between fossil and recent samples of the species. Recent specimens show slightly longer and more gracile elements as compared to fossils, which are more robust and broader, probably indicating slight but relevant changes in terms of functional morphology. These differences have originated at fast evolutionary rates, at least in the order of 200 darwins. The change is probably due to the effects of a genetic bottleneck, because no local genetic differentiation is to be expected in Mallorca in pre-human times. Other zoological groups show such fast evolutionary rates in colonization or speciation events.