As many long time turtle hobbyists can tell you, our native California desert tortoise has long been known by the scientific name Gopherus agassizii. Gopherus is a uniquely North American genus consisting of tortoises that are structurally adapted to a burrowing and digging lifestyle. Three other species of gopher tortoise are alive today - the Texas tortoise (berlandieri), the Mexican Bolson tortoise (flavomarginatus), and the Florida gopher tortoise (polyphemus). Over the years, experts in the field came to realize that the genus Gopherus really included two groups of tortoises, separable on the basis of differences in their bone and shell structure. In recognition of these differences a new genus (Scaptochelys) was proposed in 1982 that would include the desert and Texas tortoises. This new genus name was short lived, however, because it was soon pointed out that the genus nameXerobates had been proposed for the desert tortoise back in 1857, and the rules of scientific nomenclature strictly assign priority in naming to the earliest date. So, in 1984 the desert tortoise became Xerobates agassizii and the closely related Texas tortoise became X. berlandieri.
Unfortunately, name changes like these happen quite often. Usually this is because a new species or sub-species is discovered, or a new relationship is identified between already known species. On occasion, a tortoise species has apparently been named twice, as happened with Geochelone forsteni which used to be known as G. travancorica or G. forsteni depending upon where it came from. Now, thanks to the application of recently developed molecular biological techniques, the study of the classification of species and their inter-relationships is undergoing a revolution that will eventually reduce the need for many of these periodic name changes.
Instead of relying on a physical comparison of species, scientists can now compare fragments of DNA to construct "gene trees". DNA, found in the cells of all animals and plants, is the genetic blueprint that carries the genes coding for what an organism is and what it will look like. Molecular biologists have developed ways to examine DNA by cutting it into smaller fragments that can be separated by their size into characteristic patterns. Since offspring inherit their DNA from their parents the fragment patterns obtained from offspring DNA are similar to those of the parents. On the other hand, the DNA fragment patterns from unrelated individuals tend to differ. Therefore, a comparison of DNA fragment patterns can reveal how closely related two individuals or species are. Also, since species evolve over time due to the accumulation of favorable mutations in their DNA, analysis of DNA fragment patterns can give valuable information on the evolutionary relationships between species.
The first major application of these techniques to the study of chelonian biology was published recently1. Professor Avise of the University of Georgia, and Drs. Lamb and Gibbons of the Savannah River Ecology Laboratory used analysis of DNA fragment patterns to study the evolution and classification of Gopherus tortoises, placing particular emphasis on the desert tortoise.
The scientists studied a form of DNA (mitochondrial DNA) that is inherited only from the mother. They found that the desert tortoise population consists of 3 major distinct assemblages of genotypes. The first or "western" assemblage is the most wide-spread, occurring north and west of the Colorado River, and includes at least 3 sub-groups that differ only by 1 or 2 fragments. The commonest sub-group occurs throughout the Colorado and Mojave deserts in California extending into southern Nevada along the Piute Valley. Interestingly, the two other sub-groups both lie in the most northern part of the desert tortoise range. One of these sub-groups extends along the Ivanpah valley in California eastward through Nevada to extreme northwest Arizona and southern Utah. The last and smallest sub-group is restricted to the area where the borders of Arizona, Utah and Nevada meet.