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RE: C. bottae taxonomy

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Posted by: Cking at Mon Apr 21 11:05:39 2008  [ Report Abuse ] [ Email Message ] [ Show All Posts by Cking ]  

>>At times, I have found myself agreeing with Ron Nussbaum who in a 1974 paper on the species, discarded the then 3 recognized subspecies and simply lumped all Rubber Boas into a single species. Yet that arrangement overlooks the present situation (unknown at that time) where there are two discernable groupings of the species. On the one hand, there is the large morph populations characterized by large size, relatively high ventral and dorsal scale row count. In contrast, there are the dwarf morph populations all characterized by small size and with relatively lower ventral and dorsal scale row counts.>>

Phenotypically, there appear to be two groups based on your findings. A small morph and a large morph. The subspecies Nussbaum invalidated were ostensibly not based on these same phenotypes.

>>In addition, these two groupings are clustered geographically. Had such information been available in bygone years, the two groups would either have been designated as subspecies or species. However, the present mtDNA results throws a monkey wrench into that scenario. At the present, I can think of two explanation that might tend to be consistent with the mtDNA results.>>

There is something to keep in mind. Molecular evolution is independent of organismal evolution. This is a conclusion that has been nearly universally accepted after years of research pioneered by the late Allan Wilson and his students at UC Berkeley. Chance mutations at the molecular level occur at a more or less constant rate, provided that they are neutral substitutions. These rather regular changes at the molecular level is what enables the molecular clock hypothesis. The molecular clock is how Rodriguez-Robles et al. came up with their estimate of lineage divergence time of several million years among the populations of boas they studied.

Organismal evolution, such as the evolution of the large morph phenotype, is not governed by mtDNA. In other words, a change in the mtDNA is almost certainly NOT going to result in the appearance of the large morph. We must therefore look elsewhere for clues as to when and how the large morph phenotype evolved.

We do know from the mtDNA data that the boas of Kern County are geographically isolated from those in Southern California for a long period of time since they have developed different mtDNA markers. That is all that the mtDNA data can tell us: how long ago they last shared a common ancestor. mtDNA cannot tell us whether the Kern County and Southern California boas are different subspecies or different species. This is one reason Rodriguez-Robles et al. attempted to use morphological characters (instead of mtDNA) to define the Southern California populations as one species and the rest of the boas as a different species. Of course, you have in the past claimed that these characters cannot be used since they do not define these two populations. Further, you now suggest the authors were unaware of the fact that the Kern County boas are dwarf populations, just like those to the south.

Hence there really is no "conflict" between mtDNA data and "nuclear DNA" or organismal data, since they are independent of each other and they inform us of different events in the evolutionary history of the rubber boa. A synthesis of both types of data would suggest that despite the long periods of geographic isolation between Kern County and Southern California boas, there have not been any changes in either population phenotypically (while the mtDNA of both populations continue to accumulate changes through time). IOW, both populations appeared to have retained their ancestral condition of dwarfism. They have therefore undergo stasis. We can therefore safely lump these two populations into not only a single species, but a single subspecies. It is safe because your captive experiments suggest that there is no reproductive isolation between these two populations.

The southern boas therefore should not be given species status. Both the Kern County and the Southern California boas should instead be classified as C. b. umbratica.

>>1) One might argue for retention of the two species, the Southern Rubber Boa and Northern Rubber Boa with the latter being composed of two subspecies as follows: All large morph populations would comprise a northern subspecies and all other dwarf populations (expect those in the San Bernardino and San Jacinto Mts.) would constitute a southwestern subspecies of the Northern Rubber Boa.>>

As I have said above, the two species hypothesis is untenable because the Southern California subspecies and the Kern County subspecies have a common ancestor and both have changed little. Why classify a population as a different species because there have not been any evolutionary change? That would seem illogical.

>>2) A second scenario would be to have three subspecies. All large morph populations would belong to the northern subspecies, the dwarf populations in the San Bernardino and San Jacinto Mts. would belong to a southeastern subspecies, and all other dwarf population in S. Calif. would belong to a southwestern subspecies. >>

This arrangement is untenable as well. If you are familiar with the common kingsnake, Lampropeltis getulus, you would know that there are two subspecies characterized by dark coloration, namely L. g. niger and L. g. nigritus. They evolved their melanism independently of each other and they are allopatric. Hence no scientist would lump them as a single subspecies despite their similarities.

The coastal large morph boas almost certainly evolved from a small morph population that lived in western Kern County (which may have since become extinct), whereas the Sierra Nevada large morph boas probably evolved from the eastern Kern County small morph boas located directly to its south in the Sierra Nevada Mountains. The mtDNA data shows that none of the living small morph Kern County boas are genetically close to the coastal large morph population, which would suggest that the coastal population evolved its large morph phenotype first (from a small morph snake that has since become extinct, ostensibly in the area now devoid of rubber boas along the coast), and that the Sierra Nevada large morph evolved its phenotype much later from a small morph population in the southern Sierra Nevada.

>>Of course, the problem with both scenarios is that they disregard the current vogue of discarding all subspecific designations. I am not knowledgeable enough about how to interpret mtDNA results but the data presented in Javier's paper might suggest additional scenarios as well.>>

That is not the real problem. The subspecies category is useful and most biologists who are not straightjacketed by cladistic dogma will continue to use the subspecies category. The real problem with your particular 3 subspecies arrangement is that you drew the line at the incorrect places. The Southern California and Kern County dwarf boas should not be divided into two subspecies since neither of them have changed from their common ancestor, despite the long periods of geographical isolation that is evidenced from the mtDNA data. OTOH, the mtDNA data clearly shows that the coastal large morph population evolved the large phenotype before the Sierra Nevada large morph. The two populations do not meet in the southern parts of their ranges, being separated from each other by the Kern County dwarf boas, and they also do not meet in the northern Sierra Nevada, as Rodriguez-Robles et al. pointed out in their paper.

The only 3 subspecies arrangement that makes sense is a southern California-Kern County subspecies, a coastal large morph subspecies, and a Sierra Nevada large morph subspecies. Such an arrangement is in accord with the mtDNA data as well as accepted taxonomic practice of not recognizing convergently similar populations as a single subspecies.

>>Richard F. Hoyer>>

Regards, and have a good trip.


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