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John R. Holsinger and David C. Culver 
reproduce. Alternatively, low rates of ovigery may be maintained by 
group selection, with populations with excessively high reproductive 
rates becoming extinct. 
Apparently in response to low population densities, some aquatic 
species have evolved to the point where males are quite rare. We suspect 
that this is accompanied by some form of parthenogenesis (Culver and 
Holsinger 1969), but the genetics have not been studied. The isopod 
Lirceus usdagalun has a sex ratio favoring females by at least three to 
one (Estes 1978); and in the amphipod Crangonyx antennatus , male 
frequency is positively correlated with density (Dickson and Holsinger 
1981). However, male rarity is most strikingly developed in the 
amphipod genus Stygobromus . Cave populations usually have sex 
ratios of the order of 10 females to every male (Culver and Holsinger 
1969). 
Population Size and Stability 
For populations undergoing K-selection, increased efficiency, 
reduced clutch size, and the like should result in an increase in the carry- 
ing capacity, and thus increase population size. The best comparative 
data are from Poulson’s (1963) study of the amblyopsid fishes in caves 
in the Interior Low Plateaus. There is an unambiguous increase in pop- 
ulation size with increasing morphological adaptation. Considering life- 
history characteristics, however, Amblyopsis spelaea De Kay would be 
expected to have a high population size because its growth rate is low. 
In a less comprehensive study of the isopods Lirceus usdagalun and 
Caecidotea recurvata , Culver (1976) found that L. usdagalun had a lower 
carrying capacity than did the more specialized C. recurvata. 
However, there is considerable doubt that those species with the 
longest evolutionary history in caves have the largest population sizes. 
Although actual population sizes are determined by interspecific inter- 
actions as well as carrying capacity, the intensity of these interactions 
should also be under evolutionary control. Culver (1976) showed that 
the intensity of competition between any pair of species declined through 
evolutionary time but that success in competition did not increase. 
Thus, the troglophilic amphipod Gammarus minus (Form I) is much 
more common than the troglobitic amphipod Stygobromus mackini and 
the isopod Caecidotea richardsonae in the caves of the Ward Cove 
karst in Tazewell County. In the milliped genus Pseudotremia, troglo- 
philes can be as abundant as troglobites. Comparatively more special- 
ized millipeds in the genus Trichopetalum are always less common than 
those in Pseudotremia. Many troglobites, such as pseudoscorpions, are 
always rare. These observations suggest that rarity per se is advanta- 
geous, since the population avoids increases that lead to crashes and 
extinction. The most likely mechanism for evolution of rarity is group 
selection. This conclusion is supported by the fact that population 
extinctions are known to occur (Culver 1970). 
