Exceptional chromosomal mutations in a rodent population are not strongly underdominant.

The observation of karyotypic uniformity in most species has led to the widespread belief that selection limits chromosomal change. We report an unprecedented amount of chromosomal variation in a natural population of the South American marsh rat Holochilus brasiliensis. This variation consists of four distinct classes of chromosomal rearrangements: whole-arm translocations, pericentric inversions, variation in the amount of euchromatin, and variation in number and kind of supernumerary (B) chromosomes. Twenty-six karyotypes are present among 42 animals. Observations of the natural population over a 7-year period and breeding experiments with captive animals indicate that heterozygous individuals suffer no detectable reduction in fitness. This is at odds with a central assumption in current models of chromosomal speciation and provides a firm rejection of the view that selection necessarily restricts chromosomal change.