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Gary W. Dickson, et al. 
(Poulson 1964) areas. The deep-sea and cave environments probably lie 
at the extreme end of a continuum of environmental stabilities. 
Several theories relating the level of genetic variability maintained in a 
natural population to the degree of environmental heterogeneity have 
been proposed (Levins 1968; Grassle 1972; Selander and Kaufman 1973; 
Valentine 1976). The basis for these hypotheses is that increased genetic 
variation would allow individuals to be better suited to spatial and tem- 
poral heterogeneity of the environment. A population in a heterogeneous 
physical environment or a population which perceives its environment as 
coarse-grained, would be expected to maintain greater genetic variability 
than a population living under a more homogeneous regime (Nevo 1976). 
These hypotheses have received some support in experimental laboratory 
populations of Drosophila maintained under constant versus variable con- 
ditions (Powell 1971; McDonald and Ayala 1974). Genetic studies of 
deep-sea and cave faunas should permit tests with natural populations of 
the proposed correlation between genetic variability and environmental 
heterogeneity. 
Electrophoretic techniques may be employed to survey products of 
structural genes chosen without prior bias with respect to level of 
variability. Such studies have been conducted on various deep-sea inver- 
tebrates including mollusks, echinoderms and crustaceans. A recent 
review of these studies compiled by Siebenaller (1978) indicated that 
abyssal fauna contain levels of genetic variability similar to those of 
species living in other aquatic habitats. 
In one of the first genetic studies of a cave organism, Avise and Selan- 
der (1972) observed exceptionally low genetic variability in certain cave 
populations of the fish Astyanax mexicanus. Results identified an additional 
complicating factor influencing genetic variability but unrelated to selec- 
tion pressures per se : drift and founder effect in the frequently small 
cavernicole populations. Since that time several other studies of genetic 
variation in cave species have been published. Because of the possibility of 
drift lowering genetic variation in many cave populations, tests of the 
proposed relationship between environmental and genetic heterogeneity 
using cave organisms are likely to be one-sided. That is, the observation of 
normal or high levels of genetic variability would tend to refute the 
proposed correlation, while the observation of low variability could often 
be attributed to either selection pressures or stochastic events. 
Our investigation was conducted (1) to examine genetic variability in 
another cave organism, the troglobitic (i.e. obligate cave-dwelling) 
amphipod crustacean Crangonyx antennatus Packard, and (2) to summarize 
the available literature concerning genetic variation in other cave species. 
