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 ring 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). (See the species problem with Ensatina)...
The Ensatina eschscholtzii ring species 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. Ring species may be more common than realized but have been eliminated through taxonomic practice and by application of excessively strict criteria (Monahan et al., 2012). (After Kuchta and Wake, 2016)
Abstracts of Ensatina publications
All photographs Copyrighted © by Charles
or respective photographers.