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RE: C. bottae taxonomy

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Posted by: CKing at Sat Apr 19 01:17:16 2008  [ Report Abuse ] [ Email Message ] [ Show All Posts by CKing ]  
   

>>First a disclaimer- I am not a taxonomist, and I am not particularly familiar with rosy boas. I'm just throwing out some ideas here.
>>
>>You draw a connection here between size and relatedness, but this is not necessarily the case. I can see a few scenarios that would result in the conflict between MtDNA and morphological data.
>>
>>1). The adult size of these snakes is non-genetic. The 'dwarf forms' simply live in a habitat that causes them to remain small. A number of factors could lead to this: prey is scarce or of poor quality; seasonal activity is limited; larger individuals are selectively preyed upon; individuals breed at small size or early age, diverting nutrients from growth; etc. Thus the two lineages are affected by the same factors in the southern mountains. Perhaps if the southern lineage reached farther north, it would also show regional variation in size.

Environmental conditions may well be partially responsible, as the southern boas are found in mountain habitats that have cold, icy winters.

>>2). The adult size of these snakes is genetic, but small size has evolved in parallel in the southern lineage and in southern populations of the northern lineage. Similar selective factors have resulted in similar phenotypes.

That is a possibility that cannot be ruled out. The mtDNA data suggests that the species originated in the south but migrated to the north.

>>3). The converse of (2)- The adult size of these snakes is genetic, but small size is the ancestral state. Some populations of the northern lineage have evolved to larger size.

That is probably most likely. In fact, there are two allopatric northern lineages which took different routes to the north. One apparently migrated west along the mountains of Kern County and thence north along the coast ranges of coastal California, and then east across Oregon and Washington to Idaho and Utah. The other northern lineage took the Sierra Nevada route northward.

>>4.) The adult size of these snakes is genetic, and represents a primary division between a dwarf lineage and a large lineage. This is in actual conflict with the MtDNA data and could result in different taxonomic conclusions. Perhaps the MtDNA was skewed by incomplete sorting of mitochondrial lineages due to a recent divergence between these two lineages, or represents an earlier incomplete divergence between two lineages, followed by subsequent partial reintegration and redivergence.

Little chance of that because the two northern lineages almost certainly evolved their large body size independently, since they are allopatric and since they have different migration routes. South of both lineages of large morphs are the small morph snakes.

>>5.) The converse of (4)- the southern and small northern snakes are evidence of an older incomplete divergence.
>>
>>Taxonomy really has no way to deal with (4) and (5); only one split is important, the most recent one. So the following taxonomic schemes would be suitable:
>>
>>Scenario 1:
>>
>>Charina "Northern"
>>Charina "Southern"
>>
>>Scenario 2:
>>
>>Charina "Northern"
>>-----"Northern Large"
>>-----"Northern Small"
>>Charina "Southern"
>>
>>Scenario 3:
>>
>>same as 2
>>
>>Scenario 4:
>>
>>Charina "Large"
>>Charina "Small"
>>
>>Scenario 5:
>>
>>Charina "Northern"
>>Charina "Southern"

The mtDNA data suggests to me the following scenario:

Southern small,
Northern Coastal large
Northern Sierra Nevada large

Pending evidence that the two allopatric large morphs are unable to interbreed, I am assuming that they can interbreed on the basis of their morphological similarities and the lack of evidence of major obstacles to interbreeding (such as different numbers of chromosomes and polyploidy). To me then, the Rubber boas should be classified as three subspecies, not two distinct species. The three subspecies arrangement calls attention to the fact that the two northern lineages evolved independently, and also invites further study to determine whether they have become reproductively isolated or not.


   

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