Now that you have a preview of my response, you should know how ad hoc it is to argue that the common ancestor of the Colubroidea evolved venom.



As I said, powerful constrictors do not need venom. Boids are powerful constrictors, and they never evolved venom. Their near cosmopolitan distribution is testimony to their success. In troy h's scenario, the common ancestor of the Colubroidea evolved venom, then it lost the ability to constrict, then in the lineage leading to Lampropeltis and Elaphe radiata, constriction reappeared, causing the common ancestor of Elaphe radiata and Lampropeltis to abandon venom without a trace. Subsequently, E. radiata re-evolved venom from a non-venomous ancestor.



Such an implausible scenario with multiple reversals of the same character is needed if one were to account for the non-venomous nature of the ratsnakes and their descendants such as Lampropeltis. A more reasonable explanation is that the ancestor of the Colubroidea is a powerful constrictor that is not venomous. It evolved the Duvernoy's gland as a gigantic salivary gland. The postorbital position of this gland is perfect for lubrication of large prey for easy passage to the gut but not so perfect for venom delivery. If the Duvernoy's gland evolved originally as a venom gland, there should be no mucous cells in them, only serous cells. And it should have been located anteriorly so that toxins can be delivered to the prey at the earliest opportunity. Thus both the posterior position of this gland, and the anterior positioning of the fangs in viperids and elapids argue against the Duvernoy's gland having evolved originally as a venom gland. As for the biochemical similarities between the venoms of various colubroid snakes, biochemical homoplasy can explain their similarities. If the evolution of the eye in animals is any guide, pre-existing chemicals can be recruited independently in different snake lineages to serve as venom. Since the chemicals (whether they are enzymes, proteins, or products of enzyme assisted biochemical reactions) that are recruited are similar in related snake lineages, the chemical structures of the end products are also similar. If there is a way to make poison out of a particular chemical that already exists within a snake, then nature will likely make use of the same recipe again and again in different but related lineages of snakes through convergent evolution. Multiple independent evolution of venom in colubroid snakes is therefore not only possible, it is probable given the ad hoc hypotheses one would have to invent for Elaphe ["Coelognathus"] radiata to re-evolve venom from a secondarily non-venomous ancestor.

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