The Problem of defining a species
by Chuck Brown
In the Cuyamaca Mountains, Northeast of San Diego, the two of the subspecies of Ensatina, klauberi (blotched) and eschscholtzii (unblotched) are sympatric in a narrow zone, but do not interbreed. Wake et al found that in their analysis of enzymes and DNA that these two subspecies have not interbred for millions of years. However in the Palomar Mountains about 30 miles north of Cuyamaca,about 3% of the individuals of Ensatina in zones of sympatry between the two subspecies are hybrids between klauberi and eschscholtzii. Tom Devitt found that in his attempt to breed the two subspecies, both eschscholtzii and klauberi females would respond to males of the subspecies eschscholtzii. However, only klauberi females would accept klauberi males. Eschscholtzii females would not respond to klauberi males.
These observations seem to demonstrate a partial reproductive isolation between the two subspecies.Nearly all the wild hybrids that Tom has found so far have mitochondrial genes suggesting that they are the offspring of a klauberi female and an eschscholtzii male. (Devitt 2011)
A few hundred miles north in the Sierra Nevada, Brown found that about 8% of the individuals were hybrids in sympatric zones between platensis and xanthoptica. This seems to indicate that the reproductive barrier between the blotched platensis and unblotched xanthoptica subspecies is not as strong as between klauberi and eschscholtzii.
The definition of a species by Mayr is “Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” (Mayr, 1940). Ensatina with its seven distinctive subspecies does not fit clearly within that definition. Mayr states that “The nonarbitrary criterion of the category species, biologically defined, is that of the interbreeding or noninterbreeding.” Mayr does speak of difficulties of defining a species when speciation is incomplete. Such is the situation with Ensatina populations. Incipient species formation is occurring among some some of the subspecies of the genus Ensatina. (Wake 2006)
To attempt to clearly and definitively define a “species” is, I think, a mistake. It is better in the light of evolution to consider that all species are potentially in a process of speciation due to genetic variation. Thus reproductive isolation is always incomplete to a greater or lesser degree, and thus no group of populations can be considered the “perfect” example of a clearly defined “species.”
(See discussion by Wake, The Species Problem and Ensatina Taxonomy, also see the discussion by Gould, The problem with subspecies)
eschscholtzii and klauberi found under the same log
(Palomar Mnt area)
eschscholtzii x klauberi hybrid
(Palomar Mnt area)