Richard F. Hoyer wrote:

"But may I ask why cannot the original ancestor of all boa populations be the boas that now occur in the San Bernardino and San Jacinto Mts. and have remained virtually unchanged for millions of years?"

That is a very good question. Evolutionary stasis is indeed a very well known phenomenon. Darwin, for example, recognizes its existence. He writes in his book "Origin" thusly:

'We will suppose the letters A to L to represent allied genera, which lived during the Silurian epoch, and these have descended from a species which existed at an unknown anterior period. Species of three of these genera (A, F, and I) have transmitted modified descendants to the present day, represented by the fifteen genera (a14 to z14) on the uppermost horizontal line. Now all these modified descendants from a single species, are represented as related in blood or descent to the same degree; they may metaphorically be called cousins to the same millionth degree; yet they differ widely and in different degrees from each other. The forms descended from A, now broken up into two or three families, constitute a distinct order from those descended from I, also broken up into two families. Nor can the existing species, descended from A, be ranked in the same genus with the parent A; or those from I, with the parent I. But the existing genus F14 may be supposed to have been but slightly modified; and it will then rank with the parent-genus F; just as some few still living organic beings belong to Silurian genera. So that the amount or value of the differences between organic beings all related to each other in the same degree in blood, has come to be widely different."

Darwin therefore recognizes stasis in the genus he calls "F14" and he also suggests classifying species in "F14" and its ancestor "F" in the same genus. Darwin therefore not only advocates the recognition of new taxa on the basis of morphological disparity (unlike the cladists) but he also advocates the recognition of paraphyletic taxa (again unlike the cladists).

The rubber boa would appear to be a morphologically conservative species that has changed little over tens of millions of years. Its closest relative, Lichanura trivirgata, is quite similar morphologically to it, suggesting that both of these boas have remained largely unchanged over time, thus resembling each other and their common ancestor closely morphologically. The alternative is that both species of boas are quite different from what they used to be morphologically and they now look similar merely because they have evolved convergently to resemble each other because of adaptation to a similar mode of life. The second alternative is less likely since Arnold Kluge, on the basis of morphological similarity, argues that Calabaria is the closest relative of Charina and Lichanura. Either these 3 genera converged upon each other morphologically or they are morphologically conservative genera which resemble each other because of shared ancestral characters.

Since the Southern California snakes occupy a basal position in the mtDNA tree, they are thus closest to the common ancestor of all boas genealogically. That said, the mtDNA tree does show that the many extant populations of the San Jacinto and San Bernardino mountains share a common ancestor quite recently. It is as though the populations of boas suffered a catastrophic crash and these populations have only recently recovered and expanded their range from perhaps as small as a few localities or even a single locality.

Biologists like to point out that even though land vertebrates are descendants of a fish, none of the species of fishes that are living today is the direct ancestor of the land vertebrates. In the same way, none of the extant populations of San Bernardino and San Jacinto boas can be said to be the direct ancestor of the other populations of extant boas in the Sierra Nevada or the rest of this species' range. Biologists like to point out that all extant species are the descendants of a long extinct common ancestor. That said, the morphology of this extinct ancestor can often be inferred. It is indeed my inference that the common ancestor of all boas are, like the San Jacinto and San Bernardino mountain boas, small morph snakes that are also morphologically very similar to the living rubber boas occupying these mountain ranges.

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