434 
Psyche 
[September-December 
GEOGRAPHIC VARIATION AND SPECIATION 
As is the case in Antrodiaetus (Coyle, 1971), Atypoides (Coyle, 
1968 ), and probably most other burrowing mygalomorph genera 
(See, for example, Main, 1957 ; Loksa, 1966; and Forster and 
Wilton, 1968.), there is considerable geographic variation in some 
species of Aliatypus. Detailed descriptions of these geographic vari- 
ation patterns can be found in the Taxonomy section. My purpose 
here is to consider some of the causes of these patterns. 
The environmental tolerance ranges and dispersal ability of a 
species are key factors in determining what environmental conditions 
constitute barriers which can fragment and isolate its populations so 
that genetic divergence can take place. Little pertaining specifically 
to Aliatypus dispersal can be added to my earler discussion of dis- 
persal ability in antrodiaetids (Coyle, 1971). In summary, the prob- 
ability of successful colonization of distant localities by long distance 
aerial dispersal is extremely low ; aquatic rafting, short distance 
spiderling dispersal, and male wandering are probably the only im- 
portant means of dispersal under natural conditions. As in Antro- 
diaetus and Atypoides, environments with very low humidity, such 
as deserts or semiarid grasslands, are the outstanding barriers to 
dispersal and thus gene flow in Aliatypus. However, some species of 
Aliatypus, notably A. plutonis and A. torridus, are less well restricted 
by dry barriers than are most other antrodiaetids. 
The following discussions, although partly speculation, should, like 
any working hypotheses, help direct further research. Frequent refer- 
ral to Map 1 will help to understand them. 
The Central Valley of California, a semiarid grassland in its 
recent natural state, appears to be a strong barrier to gene flow 
between coastal and Sierran populations of both Aliatypus calif ornicus 
and Aliatypus erebus. The genetic discontinuity between these popu- 
lations may even be great enough to merit calling them incipient 
species. A similar situation exists in Atypoides riversi (Coyle, 1968 
and 1971). Apparently, during Pleistocene glacial periods when the 
climate was wetter and cooler, dispersal of these species occurred 
across favorable wooded parts of the Central Valley. The recent 
discovery of isolated A. calif ornicus and A. riversi populations in the 
Sutter Buttes of the Central Valley indicates that this was once part 
of such a corridor allowing gene flow across the valley. Similar dis- 
persals across the Central Valley during Pleistocene glacial periods 
are also indicated by distribution patterns of the salamander genera 
Ensatina and Taricha, which, like Aliatypus, require rather mesic 
habitats (Stebbins, 1949; Riemer, 1958). I suspect that the antro- 
