Speciation in small mammals

There are more species of small mammal (less than 1 kg) than species of large mammal, with the rodents, bats, and insectivores particularly speciose. Small mammals show greater differentiation into species at both the local and the regional level. Species of small mammal tend to occupy smaller geographic ranges than large species so that, within a geographic region the size of North America, it is common to find examples of several to many parapatric species of small mammal (often congeneric), each with essentially the same ecological needs. When species of small mammal do have large ranges, they are often subdivided into many parapatric subspecies. However, while these subspecies (like parapatric species) all utilize similar resources, they presumably each possess particular adaptations relevant to the physical and biological characteristics of the area in which they occur. It would seem reasonable to suggest that speciation in small mammals may often involve a smooth evolutionary progression within a continuous species range from locally adapted forms to subspecies to species, i.e. parapatric speciation. In fact, there are theoretical difficulties with this process and speciation in small mammals may usually require geographic isolation, in accordance with Mayr's allopatric model. The Pleistocene glaciations may have generated many such isolation events in small mammals. Speciation may occur particularly rapidly in very small isolated populations (the peripatric version of the allopatric model); there is much evidence for substantial genetic change under these circumstances in small mammals such as the house mouse Mus musculus domesticus. A fourth mode of species formation, sympatric speciation, has been suggested for bats.