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John R. Holsinger and David C. Culver 
evolved, in the tropical Hawaiian Islands at least, through adaptive 
shifts of preadapted ancestors into newly opened niches and not by 
isolation of troglophilic ancestors during climatic shifts in the Pleis- 
tocene. Although Howarth’s theory is based on data from Hawaiian 
lava tubes, which are generally much younger geologically and contain a 
significantly different food supply than limestone caves in temperate 
regions, he believes that his theory can be extended to explain to a large 
extent the evolution of terrestrial troglobites in temperate regions, 
where, as he points out, a complex geological history and glaciations 
have obscured the early history and obfuscated the previous distribution 
and evolution of troglobites. In Hawaii, cave populations may be larger 
than those of their surface relatives because the colonizable subterranean 
habitat is much larger in area than the rain forest or new lava substrate 
habitats on the surface (Howarth 1980). Howarth also suggests that an 
analogous situation may exist in limestone karst areas as well, but the 
data from our present study neither confirm nor refute this. 
In her research on cave spiders in the temperate region of southern 
Europe, Deeleman-Reinhold (1981) concluded that physical properties 
of the subterranean environment and the present areal climate have 
been the principal factors in the evolution of the high diversity of 
troglobitic species in the southwestern Yugoslavian karst. Her conclusion 
also raises a serious question about the effect of past climates on the 
evolution of terrestrial troglobites, specifically in temperate karst areas. 
One of the arguments made in the past in support of the Pleistocene 
climatic-effect theory was that terrestrial troglobites are far more 
abundant in the temperate zone than in the tropics (Barr 1968). It was 
previously assumed that terrestrial troglobites were extremely rare in 
tropical areas (see Vandel 1965b, Barr 1968, Mitchell 1969), but the 
recent discovery of rich terrestrial troglobitic faunas in Hawaii (Howarth 
1972), Jamaica (Peck 1975d), and the lowlands of Mexico and Central 
America (Reddell 1981) has proven otherwise. Because the effects of 
climatic fluctuations in the Pleistocene were probably different at low 
elevations in the tropics than in temperate zones, the Pleistocene 
climatic-effect model is questionable for areas outside temperate karst 
regions and high elevations in the tropics; thus, other explanations for 
the origin of terrestrial troglobites in the lowland tropics may be 
warranted. One such explanation is Howarth’s adaptive-shift hypothesis, 
discussed above. 
The adaptive-shift theory is attractive because it can be applied to 
all parts of the world. Therefore, it eliminates the need for different 
paradigms for different regions and is applicable to both terrestrial and 
aquatic troglobites. It is not necessarily an allopatric model, however, 
but infers a kind of parapatric speciation in which new troglobitic 
species can arise in the absence of complete physical isolation between 
epigean ancestral populations and hypogean founder populations. 
