Chromosomal speciation and phenotypic evolution in the house mouse

Chromosomal races of the house mouse (Mus musculus domesticus) in Valtellin a and Orobian Alps (Northern Italy) are known for their very fast raciation . Here we present a study using geometric morphometrics on size and shape c hanges in the skull and the mandible of three races (Orobian, Upper Valtell ina, Poschiavo) occurring in this area and forming a presumably monophyleti c group. One of the races (Upper Valtellina) went extinct recently in an ar ea of sympatry with the Poschiavo race, so that data on genetics (chromosom es and allozymes), behaviour and morphology were available to investigate c auses of phenotypic divergence during speciation with a recent extinction e vent. The evaluation of partial warp scores and the uniform component shows that morphological changes have been fast and that these races can be reco gnized on the basis of skull shape. Patterns of evolution in shape changes were visualized by combining the chromosomal phylogeny and shape space, sum marizing therefore both the phenetic and cladistic relationships. Shape cha nges follow the cladogenetic sequence depicted by chromosomal fusions. The examination of Procrustes distances shows that the different parts of the s kull evolved at different rates after speciation, with shifts in the integr ation of the various structures (olfactory, auditory, feeding, visualizatio n, etc.). Among the possible causes, aggressive behaviour was advocated for sudden changes in the shape of the skull. (C) 2001 The Linnean Society of London.