Hi, thanks for sharing the results of your very interesting breeding experiments.

You wrote:

"There are two reasonably distinct size morphs in C. bottae, a dwarf form found at the southern extreme of the species' range in Calif. and the large morph that exists in the remainder of the species' range in North America from Tulare county, Calif. northward and eastward."

The situation is slightly more complicated than that. According to the mtDNA data of Rodriguez-Robles et al., all populations of the Rubber boa share the same common ancestor. Sometime in the distant past, this ancestor gave rise to a southern and a northern lineage of rubber boas. A bit more recently the northern lineage further diverged into 2 sublineages, which Rodriguez-Robles et al. call the Northwestern and Sierra Nevada subclades.

The southern lineage is comprised entirely of dwarf morph populations, as you have said before. The problem arises when we examine the northern lineage. One of the subclades (the Northwestern subclade) consists entirely of large morph snakes, according to a reading of your past posts. The other subclade, namely the Sierra Nevada subclade, however, consists of a mixture of large and small morph populations, with the small morphs confined to the area south of Tulare County, California, again according to your past posts, and the large morphs to the north of these small morph snakes. Because of the proximity of small morph Sierra Nevada subclde snakes to the apparently ancestral Southern California populations, it is most logical to presume that these small morphs from different lineages most closely resemble each other and their common ancestor. In other words, the ancestral form of rubber boas is the small morph. The small morph Sierra Nevada snakes and the Southern California snakes have thus largely remained unchanged from their common ancestor morphologically. The large morph(s), on the other hand, probably evolved more recently.

Looking back at Rodriguez-Robles' Fig. 4, one can see that the small morph snakes from Kern County last shared a common ancestor quite recently with the large morph Sierra Nevada snakes to the north. In fact, these two morphs share a common ancestor more recently with each other than either of them does with the Northwestern subclade. This sort of evolutionary relationship suggests several possibilities:

1). The last common ancestor of the Northwestern and Sierra Nevada subclades is a small morph population, and the large morph evolved independently twice, once in the Northwestern and once in the Sierra Nevada subclades.

2). The last common ancestor of the Northwestern and Sierra Nevada subclades is a large morph, and the southern populations of the Sierra Nevada snakes subsequently underwent evolutionary reversal and re-evolved dwarfism from a large morph ancestor.

3). As you have suggested in the past, there may have been gene flow between snakes of the Central Sierra Nevada and the Bay Area snakes via the “Trans-Valley leak.” Certainly such gene flow could have brought the genes coding for large morph from the Northwestern subclade to the Sierra Nevada populations, which originally consisted entirely of small morph populations.

Possibility #3 would suggest that the large morph evolved only once, which is certainly parsimonious. However, the mtDNA data shows no evidence of such a “Trans-Valley leak.” That does not mean that it did not happen, just that there is no proof that it did. Possibility number 2 requires a reversal, which is of course not rare but perhaps a little less likely than possibility #1. Possibility number one is perhaps the most likely although one cannot completely rule out either possibilities 2 or 3.

Unfortunately I cannot help you out with the interpretation of the results from your breeding experiments, but they are interesting and they hopefully will stimulate further research into the origin and evolution of large morph(s) in the rubber boa. It is also unfortunate that Rodriguez-Robles et al.’s data cannot tell us with certainty whether the large morph evolved once or twice independently in the Northwestern and Sierra Nevada subclades. Please keep in mind that there is as yet no evidence to show whether there is one large morph or two independently evolved large morphs in the rubber boa. If the large morphs evolved independently, there is also the necessity to determine whether they are under the control of different genes (convergence) or whether they are under the control of the same genes (parallelism). The rubber boa is a very interesting species indeed.

Regards.

Reference
Rodriguez-Robles,Javier A., Glenn R. Stewart, and Theodore J. Papenfuss, 2001. Mitochondrial DNA-Based Phylogeography of North American Rubber Boas, Charina bottae (Serpentes: Boidae). Molecular Phylogenetics and Evolution, Vol. 18, No. 2, pp. 227–237

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