RATES OF MOLECULAR AND CHROMOSOMAL EVOLUTION IN SALAMANDERS.

Abstract

Changes in karyotype are receiving increasing attention from evolutionary biologists. This is in part because recent observations suggest that evolution at the organismal level is correlated more highly with karyotypic evolution than with structural gene evolution (Wilson et al., 1974b, 1975, 1977b; Prager et al., 1976; Bush et al., 1977). The salamander order Urodela is of interest in this regard because, as has long been recognized by herpetologists (D. B. Wake, pers. comm), this group is karyotypically conservative. To determine how conservative a group is, it is desirable to calculate rates of evolution. Recently, a fossil-based method was used to calculate a mean rate for 11 extant genera of salamanders (Wilson et al., 1975; Bush et al., 1977). As the authors pointed out, this method has several weaknesses, especially when applied to a group whose fossil record is so little known. For this reason, we thought it important to examine chromosomal evolution in salamanders by another method, preferably one that is not so dependent on the fossil record. Biochemical methods of comparing protein sequences can also provide estimates of divergence times among species and thus enable independent calculation of rates of chromosomal evolution (Wilson et al., 1974a,b; Wilson et al., 1977a). In this article we present the results of using a biochemical method to estimate rates of chromosomal evolution in the urodeles. The biochemical method is based on comparing the serum albumins of species whose karyotypes are known. The albumin comparisons provide an approximate estimate of the time elapsed since the species compared last shared a common ancestor. With this time, one can calculate an approximate minimum value for the mean rate at which karyotypic differences have arisen between the species compared. Two methods of analyzing the biochemical data were used and both give results in approximate agreement with those obtained by the fossil-based method, confirming that karyotypic evolution in salamanders has been slower than in frogs and very much slower than in mammals.