Chromosome Evolution and Speciation in Rodents

Abstract

Chromosomal differences are frequently associated with taxonomic differences at the species level. It has been assumed that this association in large taxonomic groups, such as the Order Rodentia, means that karyotypic divergence is causally involved both as part of population differentiation (Le. speciation) and as an adaptive component in phyletic evolution (21, 28, 147, 149). There are two proximate effects of chromosomal rearrangements that are generally in· voked to explain these observed associations, which are: (a) the meiotic effects of structural heterozygosity and the establishment of reproductive isolation; or (b) the presumed position·effect alteration of gene function and the creation of cytologically defined, coadapted gene complexes (e.g. 41). These two alternative views look at the effects of different components of fitness. The first focuses on fertility and/or fecundity in relation to the meiotic effects of rearranged chromosomes, and the second emphasizes phenotypic adaptation (i.e. temporal survivorship). Both classes of effects are intuitively appealing, and as a result, both are now commonly assumed in discussions of the role of chromosome change in speciation and adaptive evolution. However, these assumptions need to be critically reexamined.