GENETIC DIFFERENTIATION IN SPECIOSE VERSUS DEPAUPERATE PHYLADS: EVIDENCE FROM THE CALIFORNIA MINNOWS

Abstract

The systematics of cyprinid fishes native to California has had a chaotic history. Considerable controversy exists about the number of species, their placement in genera, and the evolutionary affinities among the genera (Uyeno, 1961; Hopkirk, 1973). About 16 species and 10 genera are recognized at present (Moyle, 1974). Five of these genera (Orthodon, Mylopharodon, Hesperoleucus, Lavinia, and Pogonichthys) are monotypic and restricted in distribution to a single drainage basin, the Sacramento-San Joaquin. All cyprinids in North America are thought to belong to the subfamily Leuciscinae with a single possible exception; several subfamilies are recognized in the Old World (Miller, 1959). It is commonly accepted that relatively few cyprinid fishes migrated from their center of origin in Eurasia to North America by way of a Bering land bridge during the Miocene. Therefore, most cyprinid species in North America, including the California minnows, may share a relatively recent common ancestry. Not only the lack of basic morphological diversity but also the readiness with which most American minnows hybridize (Hubbs, 1955) supports the evidence that the group has not been here long enough to develop strongly divergent lines (Miller, 1959). It has, nevertheless, been suggested that close affinities exist between certain Western cyprinid genera (Mylopharodon, Gila) and those in China and Japan (see Miller, 1965). We have analyzed patterns of genetic variability at 24 gene loci in species belonging to nine genera of cyprinids inhabiting California, using techniques of gel electrophoresis. These techniques provide considerable information to elucidate evolutionary relationships among closely related species (Avise, 1974). Our results suggest that at least four, and possibly five of the genera of California minnows are very similar in genic content and are probably of recent monophyletic origin. However, the other California minnows are less closely related, and the biochemical differences observed between these species may be typical of mean levels of divergence between other North American cyprinids. Models are considered which describe amounts of genetic differentiation expected between species within a group. One model assumes that genetic distance between species is proportional to the time since they shared a common ancestor; the other model assumes that genetic distance is proportional to the number of cladogenetic events (speciations) in the evolutionary history of the group. Those models lead to distinct predictions of mean amounts of genetic distance between species in speciesdiverse versus species-depauperate phylads of equal evolutionary age. Our data on the California minnows suggest that mean time since divergence from a common ancestor is more important than mean number of cladogenetic events in the evolutionary history of a group of species as a predictor of levels of biochemical differentiation.