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The genus Lampropeltis

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Posted by: CKing at Wed Jun 11 01:59:07 2008  [ Report Abuse ] [ Email Message ] [ Show All Posts by CKing ]  

I came across a very interesting paper on my favorite genus. It can be found in the link below article.pdf

Robert W. Bryson Jr. a,¤, Jennifer Pastorini b,c, Frank T. Burbrink d, Michael R.J. Forstner, 2007. A phylogeny of the Lampropeltis mexicana complex (Serpentes: Colubridae) based on mitochondrial DNA sequences suggests evidence for species-level polyphyly within Lampropeltis. Molecular Phylogenetics and Evolution 43:674-684

It is a very interesting paper indeed because it sheds a good deal of light on the relationships within this genus. Here is my interpretation of the results:

The genus Lampropeltis evolved from a species of New World Elaphe, which migrated from the Old World. The ancestral species in the genus Lampropeltis was a ringed tricolor and this species was very successful, as evidenced by the polytomy at the base of the tree uniting the known species of Lampropeltis and of course the distribution and diversity of this genus. The polytomy signifies that very early in the evolution of this genus, it had already evolved rapidly into several different mtDNA branches or lineages (no doubt by rapid range extension). One of these lineages is L. zonata, and a second lineage is L. pyromelana (and the obviously closely related “L. webbi”).

A third lineage is the L. triangulum-L. mexicana-L. ruthveni-L. alterna complex and their descendants. Among the descendants of this complex are such species as L. calligaster, L. getulus, Stilosoma extenuatum and Cemophora coccinea (based on prior studies). That means the genus Lampropeltis is “paraphyletic.” Of course paraphyletic taxa bothers the cladists endlessly, so we may one day see the genus Lampropeltis split up into several different genera, or else Stilosoma and Cemophora will probably be transferred to Lampropeltis by some cladist who cannot help himself/herself. Personally, I am not bothered by paraphyletic taxa, since they are the natural consequence of the process of evolution, as herpetologist Robert L. Carroll points out. When a new species evolves and is sufficiently different from its parental genus, it should be placed in a new genus of its own. Otherwise all life on earth must be placed in a single guenus. Whenever a new genus is named, the parental genus becomes “paraphyletic” because it no longer consists of all the descendants of a single common ancestor. Paraphyletic taxa never bothered Charles Darwin or his followers, and they did not even have a name for such taxa. They simply consider paraphyletic taxa monophyletic and valid. Enough digression and venting against the cladists for now.

The one big surprise in this study is that L. mexicana mexicana, L. ruthveni and L. m. greeri are not closely related to L. alterna, as previous researchers had assumed. Instead, it appears that L. alterna and the closely related “L. m. thayeri” are only convergently similar to the L. m. mexicana-L. ruthveni-L. m. greeri group. Both of these groups apparently evolved independently from separate subspecies of L. triangulum. The L. m. mexican-ruthveni-greeri group is closely related to L. t. arcifera, L. t. conanti and L. t. campbelli, whereas L. alterna and “L. m. thayeri” are closely allied with L. t. celaenops and L. t. gentilis. Bryson et al. placed the two different groups of L. triangulum on different ends of their diagram, making it look as though L. triangulum is an unnatural polyphyletic species. However, if we put these two groups next to each other, we will see that they do share a common ancestor, and this common ancestor is part of the polytomy at the base of the genus Lampropeltis.

The bottom line:

L. mexicana mexicana is not conspecific with "L. m. thayeri." So thayeri should probably be removed from the synonymy of L. mexicana. L. triangulum is paraphyletic. Big deal! It bothers the cladists but it does not bother me or the Darwinians.

L. webbi is part of the lineage that also includes L. p. pyromelana and L. p. knoblochi. Again this is not a big surprise. In fact, L. webbi should be synonymized with L. pyromelana IMHO. L. webbi should not have been recognized in the first place.

Bryson et al.’s paper also corroborates the results of the paper by Rodriguez-Robles et al. concerning L. zonata. It appears that L. z. multicincta is a descendant of L. z. multifasciata and L. z. parvirubra and L. z. pulchra are closely related (systematists call them ‘sister taxa’). So, I think their data is pretty reliable. I do think they themselves have trouble dealing it though.

Bryson et al. suggest that there may be interbreeding between L. triangulum and the L. alterna-L. thayeri complex that could explain their results. I doubt that. I think their data can be simply explained by a separate origin of L. alterna from one subspecies of L. triangulum, and L. mexicana-greeri-ruthveni from another subspecies of L. triangulum. L. triangulum appears to be very much a single species, albeit a paraphyletic one, since quite a few taxa have "budded" off this ancient species over time. The many descendant species of L. triangulum include Stilosoma extenuatum, Cemophora coccinea, L. getulus, L. mexicana , L. m. greeri, L. thayeri, L. alterna, and L. calligaster, for now. Who knows, there could be others.

Bryson et al. write: "Our phylogenies also indicate that the genus Lampropeltis appears paraphyletic with respect to Stilosoma. All analyses support a sister relationship between the getula clade and S. extenuatum, consistent with previous studies (Dowling and Maxson, 1990; Rodríguez-Robles and De Jesús-Escobar, 1999). Given the morphological differences between these two species, this relationship is unexpected."

Well, as Darwin himself pointed out, evolutionary rates are different in different lineages. Some lineages simply evolve more quickly than others. Stilosoma evolved at a much faster rate than L. triangulum. This should not come as a surprise whatsoever. Since the rates of evolution are different, it is necessary to formally recognize these differences in our classifications, even if that means recognizing paraphyletic groups, which are unfortunately not something a cladist finds acceptable. That is unfortunate indeed.


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