60 Donald A. Merkle 



have been observed in isolated populations of other species (Avise 

 1976). Fixation of Xdh-l b in the Sea Shore State Park population also 

 appears to be in progress. Additionally, the presence of Lap-l a in only 

 the Hopewell and Newport News populations indicates the close genetic 

 relationship between them. This suggests that the populations of A. pis- 

 civorus north of the James River are derived from populations in the 

 vicinity of Hopewell rather than farther down the river. This hypothesis 

 is supported by other species of snakes with distributions similar to that 

 of the cottonmouth. Nerodia taxispilota occurs on the Peninsula, but 

 does not extend to its lower end. The single Virginia record for Regina 

 r. rigida is also from the uppermost reaches of the Peninsula. Whether 

 this represents a relict population or a short-lived introduction is prob- 

 lematical. While these species would be able to cross the James River at 

 its widest expanse, they are absent along the river in its lower reaches. 



The distribution of the species least likely to cross large expanses of 

 water, C. horridus, lends further credence to the hypothesis. The timber 

 rattlesnake has a more extensive distribution on the Peninsula than does 

 the cottonmouth, but it also is not recorded from the counties directly 

 below the James River. Yet there are records for this species from 

 Prince George County, very near the Hopewell population of A. pisciv- 

 orus. Both N. taxispilota and N. e. erythrogaster also persist in this 

 same general region. 



The elimination of intervening populations for these species may be 

 a result of climatic factors, at least in the case of A. piscivorus. It 

 appears that the distributional range of this species in Virginia is con- 

 tracting. Richard Hoffman (in Russ 1973) reported that cottonmouths 

 were once common east of the Fall Line in Virginia, but are now only 

 rarely found in many areas. Today, even in the Dismal Swamp, the 

 species is so rare that the Park Service will not issue permits to collect 

 this species. Many areas that once supported large populations no 

 longer do so, and it appears that the range of this species is being 

 pushed to the southeast. Blem (1981) felt that the record cold winters 

 during the last decade may have had a decimating effect on the survival 

 of this species at the northern edge of its range. 



In summary, it appears that the levels of genetic variation observed 

 in six Virginia populations of A. piscivorus are lower than those 

 reported for other snakes. This species and several others of the Coastal 

 Plain probably had more extensive distributions in the past, but appear 

 now to be undergoing range contractions. Virginia cottonmouth popu- 

 lations that still seem to be thriving also display the highest genetic 

 variation. 



