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Psyche 
[September-December 
represent the kind of semi-isolated, genetically divergent, peripheral 
populations found in many species. Aliatypus thornpsoni variant 
populations are also at the periphery of the species range, where 
suitable habitats are probably uncommon and semi-isolated. The 
Tehachapi Mountains apparently provide (or recently provided) an 
east- west corridor of favorable habitats for the dispersal of A. janus 
and A. thornpsoni between the southern end of the Sierra Nevada 
Mountains and the coastal mountain ranges. The morphologically 
divergent nature of the A. thornpsoni samples at Tehachapi and in 
the southern end of the Sierra Nevada Mountains indicates that this 
corridor is not currently supporting much dispersal. 
The phenotypic differences between the two known Aliatypus 
isolatus populations in Arizona are almost certainly caused by the 
disruption of gene flow after the recent (Wisconsin) glacial period 
as desert and grassland barriers expanded all over the Southwest to 
isolate various mesic mountain habitats. Pollen analyses (Martin 
and Mehringer, 1965) demonstrate that during the Wisconsin glacial 
period (which apparently lasted in that area until about 13,000 years 
ago), woodland and forest habitats favorable for A. isolatus extended 
continuously throughout western and northern Arizona. Thus the 
similar geographic variation patterns in A. isolatus and Antrodiaetus 
apachecus (Coyle, 1971) probably have a common cause. 
The extreme similarity of allopatric Aliatypus janus and Aliatypus 
isolatus , when viewed with Southwest pollen analyses (Martin and 
Mehringer, 1965) in mind, leads to the conclusion that these two 
species were formed when a recent interglacial expansion of the 
Sonoran and Great Basin Deserts severed a previously widespread 
ancestral population. Convincing evidence that 17,000 to 23,000 
years ago (during the Wisconsin glacial period) woodland extended 
continuously from current A . isolatus localities to the present range 
of A. janus , strongly indicates that these sister species may be only 
15,000 years or so old. Indeed, it is possible that genetic divergence 
has not even progressed far enough for the development of repro- 
ductive isolating mechanisms. 
There are three other pairs of closely related Aliatypus species — 
A. janus and A. aquilonius , A. calif ornicus and A . gnornuSj and A. 
erebus and A. trophonius — which are not as similar as A. janus 
is to A. isolatus. It is possible that each of these pairs originated 
from a trio of ancestral species fragmented by arid barriers, such as 
the present Central Valley, during an earlier Pleistocene interglacial. 
Interestingly, each of these pairs consists of a large and a small species. 
