Speciation in Progress
A Classic Example of Darwinian Evolution
Ensatina eschscholtzi is a lungless salamander of the family Plethodontidae. The distribution of this species is from British Columbia in Canada, through Washington, Oregon, California and into Baja California of Mexico. Presently, seven subspecies are recognized, and all occur in California. The subspecies are eschscholtzi, xanthoptica, oregonensis, picta, platensis, croceater and klauberi. (Stebbins, 1949) There is a considerable variation in color patterns among the subspecies. Populations of the subspecies form a ring around the Great Central Valley of California. In order of their geographic distribution, eschscholtzi occurs in the coastal mountains of Southern California and range northward to Monterey Bay area and intergrades into the next subspecies xanthoptica. Xanthoptica is located in the San Francisco Bay area and in turn, intergrades with oregonensis near Santa Rosa to the north. Oregonensis occurs from Santa Rosa northward through Oregon and Washington into British Columbia. In Northwestern California oregonensis intergrades with picta and to the east near Mount Lassen, oregonensis intergrades with platensis. Platensis is found all along the Sierra Nevada from Mount Lassen to the Greenhorn Mountains southeast of Bakersfield where it intergrades with the subspecies croceater. Croceater occurs in the Tehachapi Mountains due south of Bakersfield. There is a gap between croceater and the next populations of Ensatina, the croceater-klauberi intergrades in the San Gorgonio Mountain area east of San Bernardino. To the south, in the mountains northeast and east of San Diego occurs the subspecies klauberi.
What is most interesting about this species of salamander, is that the two southern most subspecies, eschscholtzi and klauberi, meet in several locations. Near Mount Palomar, these two subspecies meet in a very narrow zone and hybridize infrequently. (Brown, 1974) To the south near Cuyamaca State Park, klauberi and eschscholtzi meet and apparently fail to interbreed under natural conditions even though they are narrowly sympatric. In fact, by analyzing electrophoretic separations of selected enzymes and studying DNA patterns, the two subspecies klauberi and eschscholtzi are different species by every definition. (Wake, Yanev and Brown, 1986) This poses a very interesting problem. Should the species Ensatina eschscholtzi be split into two or more species, or be considered a single species? If the species is to be split, where does one draw the line?
Research on enzymes, nuclear DNA and mitochondrial DNA, are now being conducted by David Wake at the Museum of Vertebrate Zoology in Berkeley. These studies by Wake, show evidence in support of the idea that Ensatina eschscholtzi is a species complex that is now breaking up into two or more species. While the hypotheses by Stebbins, 1949, for the Ensatina complex is supported by recent works, the original idea is too simple. Differentiation is greater than originally conceived, thus an argument could be made for dividing Ensatina into several species. However, incipient species formation is in its early stages and thus species borders and distinctions remain unclear (Wake, 1997). The biological complexity of Ensatina argues against a simple taxonomic resolution because the evolutionary realities of diversification in old and persistent complexes require compromises if Linnaean taxonomies are to be used. A preferred taxonomy is one that clarifies the evolutionary relationships among the components and that highlights, rather than obscures, the complex interactions of the past and the present. A new taxonomy may be required when studies in progress are concluded, for the present the Ensatina complex will be recognized as a single taxonomic species (Wake and Schneider, 1998).
The Ensatina complex appears to be a classical example of Darwinian evolution by gradualism; an accumulation of micro mutations that is now leading to the formation of new species.
All photographs Copyrighted © by Charles