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Figure 4.7 Biogeographic analysis of U.S. cave Collembola by Christiansen and Culver
(1987). (a) Generalized track I; (b) generalized track II; (c) geographic distribution of troglo-
morphy of cave Entomobryinae (solid black: most troglomorphic species; dots: least troglo-
morphic species).
Christiansen and Culver (1987) analyzed the geographic distribution of ninety
species. They found two generalized tracks in the United States. One runs in
straight line east-west along the 37° and 38° parallels, generally bending north-
ward in West Virginia and Virginia ( fig. 4.7a ). Another generalized track starts in
southeastern Missouri and curves first south and then north, ending in south cent-
ral Pennsylvania ( fig. 4.7b ). According to the authors, both tracks are very simil-
ar and may be variants of a single generalized track, although they treated them
as distinct based on the different taxa involved in each of them. For Mexico, the
Caribbean, and South America the patterns were much less clear, and no gener-
alized tracks could be identified.
Christiansen and Culver (1987) also analyzed the levels of troglomorphy (char-
acters that change progressively with increasing time in caves) exhibited by some
of the taxa analyzed and correlated them with the geographic range. They found
that smaller geographic ranges were associated with increased troglomorphy, and
they considered this evidence of dispersal from a center of novelty and a series of
more troglomorphic species spreading out. Distribution of single lineages showed
peripheral or central evolutionary novelties, but examination at a larger scale ( fig.
4.7c ) did not reflect a clear trend for the most advanced or most primitive taxa to
be peripheral or central. The authors concluded that the panbiogeographic meth-
od was useful to analyze cave Collembola in the restricted sense of providing a
simple technique to look for evidence of past dispersal events.
 
 
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