I should also mention that the question of whether or not H.n.gloydi is a valid taxonomic entity is not completely closed. If you examine the locality data for all the specimens in the museums you will find an absence of specimens in much of central and north central Texas. So it could be argued that the southeastern populations of Heterodon nasicus in Texas could be genetically isolated.
My own opinion is that the appearance of isolation may be mostly a result of lack of sampling. The southeastern portion of Texas is ideal because of the loose, coarse, marine deposits but there are some major corridors that connect this region of Texas to the northwestern and northern portions of Texas, where H. nasicus is again fairly common,... the several large rivers like the brazos, colorado, etc. extend from one area to the other. The rivers themselves are not the corridors. Instead, these rivers have large plains of Alluvial deposits along their banks that are excellent habitat for Heterodon nasicus and H. platyrhinos alike.
It is my belief that these rivers serve as a corridor for dispersal and genetic exchange. The clinal pattern of dorsal blotch change is a bit broader in this region which is something I would expect in a region where genetic exchange was not as extensive as it is in the rest of the population.
Anyway, this question is not likely to be fully answered until someone does an extensive genetic sampling of specimens throughout this area and subject them to genetic analysis. However, I believe they will come to similar conclusions that I and others have based on morphology and biogeography.
Curtis Eckerman
My Homepage
