KARYOTYPE EVOLUTION - EVIDENCE FROM THE COMMON SHREW (SOREX-ARANEUS L)

The common shrew is an exceptionally variable animal with respect to its chromosomes. We here review the evidence from the species concerning general questions of karyotype evolution. It appears as if chromosomal heterozygotes always are at a fitness disadvantage and that there is no exceptional amount of inbreeding that may alleviate this heterozygous effect. No other factor has been found that convincingly explain the spread of new chromosomal types and the great karyotypic variability in the species. The species is split into a number of races with incompatible karyotypes. Studies of the hybrid zones between such karyotypic races show that gene flow goes through the chromosomal barrier despite the fact that the chromosomal hybrid zones are very sharp. It has also been shown that secondary evolutionary processes in the hybrid zones tend to decrease the genetic isolation between the races involved. There is, thus, no evidence from the common shrew which indicates that simple chromosome mutations play a special role in speciation. Copyright © 1990, Wiley Blackwell. All rights reserved