Morphological effects of low temperatures during the embryonic development of the garter snake, Thamnophis elegans

I analyzed geographic variation within Florida for 15 species of snakes. Contour maps were machine-produced for each of over 200 morphologic and 17 environmental variables. Factor analysis of the maps showed that seven major patterns of geographic variation account for over 60% of the total variation. Each of the patterns of geographic variation can be explained in terms of natural selection and gene-flow characteristics in past or present environments. Disjunct populations showing phenetic similarities are the result of an earlier widespread phenotype followed by differentiation in geographically intermediate regions. Recourse to land bridge hypotheses and retrogressive evolution are not necessary to explain polytopic phenotypes. The geographic localities of primitive species or primitive characteristics within a species are not the centers of origin, but are merely regions in which evolution has proceeded relatively more slowly. A north-south cline on the Florida peninsula is the most obvious pattern of geographic variation, common to all species examined for at least some characters. Most species examined showed aspects of a Suwannee Straits pattern with dramatic character-state shifts occurring in the region of the present Suwannee River.

Section: Discussionmentioning

confidence: 87%

Patterns of geographic variation in Florida snakes

Christman

1980

“…Most of these high-altitude populations have mean ventral numbers markedly different from those in the geographically adjacent lowlands. Rather than decreasing, as one might predict on the basis of temperature-scutellation studies on T. elegans ( Fox, 1948;Fox, Gordon, and Fox, 1961), the number of ventrals increases. Furthermore, in certain areas the amount of increase seems roughly correlated with the amount of increase in altitude.…”

Section: Geographic Variationmentioning

confidence: 87%

“…While some of these populations can be distinguished by differences in scale counts •as well as in color patterns I have placed more emphasis on color because it is usually less variable over wide geographic areas, a common phenomenon in the genus Thmnnophis ( Fitch, 1941, and others). Fox, Gordon, and Fox ( 1961) have sliown that color patterns in individual T. elegans subjected to low temperatures during embryonic development do not differ from those of animals that deyeloped at normal temperatures, pehile ventral and subeaudal numbers differ stfikingly between the two groups.…”

Section: Taxonomymentioning

confidence: 98%

The Colubrid Snake Genus Thamnophis: A Revision of the Sauritus Group

Rossman

1963

The Sauritus group of the gartersnakes is composed of two polytypic species, Thamnophis sauritus (Linnaeus) and Thamnophis proximus (Say), commonly called the ribbonsnakes. Both species vary geographically in number of ventrals, number of subcaudals, relative tail length, and several features of the color pattern. Supralabial number is subject to geographic variation in T. sauritus but not in T. proximus. Most of the geographic variation is discordant, transition zones in one character seldom corresponding with those of other characters. As color pattern fluctuates less locally than do the meristic characters, it is more reliable in defining geographic races. Four subspecies of T. sauritus are recognized, two of them previously undescribed ( T. s. septentrionalis of the Great Lakes region, northern New England, and Nova Scotia; T. s. nitae of northwestern peninsular Florida). Of six races of T. proximus four are new (T. p. orarius of coastal Louisiana, Texas, and northern Tamaulipas; T. p. rubrilineatus of the Edwards Plateau region of central Texas; T. p. diabolicus of New Mexico, trans-Pecos Texas, Coahuila, and Nuevo Leon; T. p. alpinus of the Chiapas Highlands in southern Mexico). The evolutionary history of this species group and the trends within each species are discussed. Members of the group are highly specialized and occupy an advanced position within the genus.

“…Though laboratory studies have shown tempetature increase'to be an effective mutagen in Drosophila, it is not wise to demand a genetic basis for all cases of intraspecific variation in snakes. Scutellation variations similar to some in Rhadinaea have been induced experimentally in Thamnophis elegans by lowering environmental temperatures during embryonic growth (Fox et al, 1961). Possibly R. #avilata is also showing phenotypic susceptibility to environmental change, but the implications of the Thamnophis study are not easily applied.…”

Section: Supralabialsmentioning

confidence: 99%

“…An increase to 10 plates is the result of a division in one of the frst several plates (second to fourth, apparently) or in one of the last. Fox et al (1961) report a significant correlation between low supra-and infralabial counts and low environmental temperatures during embryonic development in the snake Thamnophis elegans.…”

mentioning

confidence: 91%

The Pine Woods Snake, Rhadinaea flavilata (Cope)

Myers

1967

Rhadinaea flavitata (Cope, 1871) is a small colubrid snake inhabiting a narrow coastal range in the southeastern United States. Its distribution closely approximates that of the low pine fiatwoods, which constitute its principal habitat. A few individuals have been found in other situations, especially in Florida hammocks and on coastal islands. The species is most commonly encountered in pine logs and stumps during March and April; with the approach of warmer weather and drier conditions in May and June, Rhadinaea and other small snakes become more difficult to find. R. flavilata feeds on small amphibians, lizards, and perhaps snakes; literature records referring to predation on insects are questioned. The pine woods snake kills or incapacitates some of its prey with a venom introduced into wounds made by enlarged rear maxillary teeth. Other snakes are likely its main predators and food competitors. R. flavilata lays two to four elongate eggs; the period of egg deposition is speculated to extend from May into August, although it probably is not so extensive in any given season; the natural nest is unknown. There is slight statistical evidence of unbalanced sex ratios in some populations. Structural variation was observed in many characters, and for discussion is classified as intrapopulational variation (including ontogenetic, sexual, and uncorrelated or "individuial" variation) and interpopulational (geographic) variation, the latter being compounded from the former. No geographic races are recognized in this species. Structurally the pine woods snake is a rather generalized Rhadinaea. Its closest living relative is R. laureata of the highlands of western Mexico. Fossils show that R. flavilata, or a genetic predecessor, had arrived in the southeastern United States at least by the third (Illinoian) glacial stage of the Pleistocene. The lack of marked geographic variation in this species can be explained by the relatively recent (postglacial) dispersal of a homogeneous Floridian stock throughout a relatively uniform pine-flatwoods habitat. Environment, nonselective genetic mechanisms, and natural selection are considered briefly in terms of the evolution of geographic variation. A seemingly primitive color pattern is retained at the northern limits of the range; the southern populations are characterized by loss of pattern and by increased variability (often anomalous) in scutellation. It is inferred that southern populations, being most influenced by a warming post-glacial climate, are evolving faster than northern populations.