Chromosomal evolution in small mammals (Insectivora, Chiroptera, Rodentia)

Extensive descriptive, comparative, and experimental research on the chromosomes of natural populations of small mammals has been conducted in the last 50 years. These studies have revealed a surprisingly large amount of karyotypic variation within and between individuals, populations, species, and higher taxa. In the Palaearctic region, the karyotypes of 80 to 90% of the species of insectivores, bats and rodents have already been described, and almost all European species belonging to these orders have been examined. More than 40 cryptic species of small mammals with a unique karyotype have been described in the Palaearctic region, including 24 species in Europe. A polymorphic or polytypic karyotype was found in 118 Palaearctic and 42 European species. This high degree of intraspecific karyotypic variation has resulted in problems in the naming of various chromosomal races, since the subspecies is clearly not the appropriate category for this purpose. The driving forces of karyotypic evolution may be found either in selection or drift acting at the organismal level, or in the internal processes occurring within the cell. The forces acting at the organismal level are based on either negative heterosis of chromosomal rearrangements or on the altered pattern of gene expression resulting from karyotypic repatterning. Little evidence for the direct adaptive nature of chromosomal alterations has been presented up to now and the significance of this factor remains unclear. Chromosomal change is, however, obviously correlated with speciation and divergent evolution, even if karyotypic alterations in certain lineages need not be directly related to the formation of a reproductive barrier. Chromosomal studies are still an important tool to record and describe biological diversity, and often represent a simple and indispensable method for identification of various taxa.