Posted by:
CKing
at Sat Nov 1 04:59:01 2003 [ Email Message ] [ Show All Posts by CKing ]
troy h wrote:
as Dr. Fry pointed out, the loss of venom (or toxins if you prefer) only occurs once among Colubrids - in the lampropeltines . . . and a reversal in one segment of this lineage (Coelognathus) is not a giant leap. It is far more parsimonious (therefore vastly more probable) that venoms evolved once (as is supported by shared chemical composition of many venoms) and that one group lost the venoms (once) and for it to reappear (once) in a subset of that group. Your hypothesis, that venoms evolved many times, is not supported by chemical analysis of venoms (as Dr. Fry pointed out) or by the principle of parsimony.
Further, the evidence that Colubrids lost constriction and that it has re-evolved in many colubrid groups, while not the most parsimonious explanation, is well supported by evidence from the widely varying patterns of constriction found in Colubrids - all primitive constrictors constrict in precisely the same pattern while advanced constrictors use a wide variety of constriction patterns.
The evidence is strong, follow the evidence. Or stick to dogma - your option.
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Troy h later gave the following reference as evidence that colubrid snkes lost constriction and constricting behavior then re-evolved in many colurbid groups:
Greene, H.W. & Burghardt, G.M. (1978) Behavior and phylogeny: constriction in ancient and modern snakes. Science 200:74-77
Let’s examine the evidence. Greene and Burghardt state that “There are four possible origins of similar phenotypes in different taxa.” These 4 possibilities are (i) chance, (ii) similar experiences in the lives of individual animals, (iii) convergence, (iv) phylogenetic continuity. They believe that the first 3 can be falsified for the evolution of constricting behavior. That means they believe that if two snakes constrict in a similar fashion, then their phenotypic similarities are due to common ancestry. In footnote number 14, on page 76, they state: “Among colubrids, some coils of Arizona and Lampropeltis resembled those of boids.... A published drawing of Regina alleni (Colubridae) suggests that this species might also at least sometimes use the boid pattern.”
That means, according to Greene and Burghardt, the similarities between Regina, Arizona, Lampropeltis and the boids are due to “phylogenetic continuity” since they had “falsified” the other 3 possibilities for these similarities. If there is phylogenetic continuity in constriction behavior between the boids and colubrids, then needless to say there has not been any loss or discontinuity in this behavior in Regina, Arizona, Lampropeltis, in the common ancestor of these snakes, or for that matter in the common ancestor of the Colubroidea. Lampropeltis, Arizona and Regina therefore have retained the constricting behavior that has apparently evolved in the common ancestor of all snakes, which lived before the end of the Cretaceous.
I agree with troy h that the evidence is strong, not for the loss and re-acquisition of constricting behavior in colubrid snakes but for “phylogenetic continuity” in constricting behavior from primitive to adavanced snakes. If there is loss of constriction among colubrids, it almost certainly occurred independently multiple times, but not in the common ancestor of the colubrids.
If the common ancestor of the coluroid snakes never lost constriction, then, like Lampropelits and Arizona, this ancestor is probably non-venomous. Powerful constrictors such as boids and Lampropeltis simply do not need venom to be successful. Judging from the holarctic distribution of the ratsnakes and the distribution of the genus Lampropelis (from Canada to northern South America) and the distribution of boids, these snakes are indeed very successful.
If the colubrid ancestor did not lose the ability to constrict, then there is no adaptive reason for it to evolve venom and then lose it, as it has been claimed to have happened in the lampropeltine snakes. It is far more parsimonious for Lampropeltis to have never lost constriction and never evolved venom then for it to have evolved from an ancestor that had lost the ability to constrict and thence evolved venom, and then re-evolve constriction and finally lose the venom. Evolution is not parsimonious, but it is not ad hoc either.
Mayr and Ashlock write:
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Evaluation of Homoplasy
When there is discordance among characters because of homoplasy, characters should be weighted and ranked according to their quality. The following criteria are helpful in determining the character to which the highest weight should be assigned (Bock 1977: 890).
1 It is a complex feature rather than a simple one because the probability of evolving independently two or more times is considerably lower for a complex than it is for a simple feature.
2 It is a new feature rather than the reduction or loss of a feature because the probability of a new feature evolving independently two or more times is low while reduction or loss of a feature may occur several times independently.
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It is more likely for constriction to have been lost independently multiple times in many colubrid lineages than for some (such as Lampropeltis) to have lost it, evolve venom, reevolve constriction and then lost venom without a trace.
Venom evolved once?
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 Did the colubrid ancestor lost the ability to constrict? - CKing, Sat Nov 1 04:59:01 2003
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