Genetic Variation 
127 
habitats. Values of genic heterozygosity in many cavernicolous popula- 
tions are comparable to those of species from epigean environments (Nevo 
1978). However, a cautionary note is required. Studies on troglobitic 
organisms are still few in number, and generally have included only a 
small number of loci. Results are not yet sufficient to eliminate the 
possibility of a mild quantitative decrease in genic variation in cavern- 
icolous populations. It does appear very likely that the genetic response of 
troglobitic organisms to their environment (at least as evidenced by level 
of genetic variability) is not qualitatively different from that of other 
species. Analogous conclusions have been reached from studies of 
organisms inhabiting abyssal areas (Siebenaller 1978; Costa and Bisol 
1978). 
The genetic variability observed in populations inhabiting stable en- 
vironments (i.e. caves and deep-sea) may have different origins. 
Siebenaller (1978) proposed that the relatively high variability observed 
in many deep-sea invertebrates could be due to the presence of large pop- 
ulation sizes and to interactions predicted by the time, population size 
and divergence hypothesis of Soule (1976). In cave organisms, species and 
population numbers are normally smaller than in most faunal counter- 
parts living in other habitats, including abyssal areas (Poulson 1964). 
The normal levels of genetic variability in these subterranean species 
could be due to a process described as ecological release (Avise and Selan- 
der 1972). In the generally depauperate cave communities, troglobites 
could expand their niches and exploit resources which would normally be 
shared by two or more species. Relatively high levels of genetic variability 
might be maintained under these conditions. 
If genetic variation is not neutral with respect to fitness, normal levels 
of genetic variability in deep-sea and cave organisms could also be main- 
tained by selection due to spatial or temporal heterogeneity. Although 
caves and the deep-sea are considered more stable than most other 
aquatic and terrestrial areas, physical habitat variability and seasonal 
fluctuations in certain parameters are present (Poulson and White 1969; 
Menzies et al. 1973). It is possible that the threshold of environmental 
heterogeneity below which significant genetic variability is no longer 
favored is still lower than the level of environmental variation in the 
proverbially stable environments of caves and the deep-sea. 
ACKNOWLEDGMENTS . — We are grateful to Mrs. Eleanor Berry, 
owner of Molly Wagle Cave, and to other cave owners in Lee County, 
Virginia, for their cooperation in allowing us access to their property. In 
addition, we thank Karen L. Stueck, Raymond R. White, Donald G. 
Buth and an anonymous reviewer for providing helpful comments and 
