52 
Psyche 
[March 
Various explanations for the phenomenon of neatly allopatric 
sister species have recently been considered by, for example, Mayr 
(1963: 81-82), Remington (1968), and Selander (1969: 2 15-2 19). 
Mayr feels that interpretations — like the one offered here for P. 
oileus and P. philetas — involving differences in adaptation to sharply 
breaking variables of the environment, probably apply in a majority 
of cases. Remington and Selander, in dissimilar contexts, both 
emphasize interpretations involving competitive exclusion, though 
Selander implies that competitive exclusion may be pertinent in a 
minority of cases. There is general agreement that persistent allopatry 
of closely related species often poses intricate evolutionary problems 
that are hard to analyze and interpret with confidence. 
Summary 
Pyrgus oileus and P. philetas , though closely related, are incontest- 
ably different species — some current opinion to the contrary notwith- 
standing. Sympatric in part of Mexico, these morphologically distinct, 
multivoltine species become allopatric toward their northern distribu- 
tional limits in the southern United States. There, P. oileus of eastern, 
moister environments and P. philetas of western, drier environments 
approach each other or meet in central Texas, and overlap slightly 
in space (and fully in time) in southcentral Texas, without hybridiz- 
ing. Although competitive exclusion could be suggested, it appears 
more likely that these species are limited (either directly or indirectly) 
by an abiotic factor, humidity. 
Acknowledgements 
We thank F. H. Rindge and W. D. Field for access to Pyrgus 
material in their care at the American Museum of Natural History 
and the United States National Museum, respectively; B. and K. 
Mather for lending Mississippi specimens of P. oileus , H. A. Free- 
man for lending peripheral Texas specimens of P. oileus , and J. W. 
Tilden for lending all specimens of P. oileus and P. philetas in his 
collection; and R. G. Gillmor for drawing the genitalia figures. 
This research was supported in part by National Science Foundation 
grant GB 5935 to J. M. Burns. 
Literature Cited 
Bottimer, L. J. 
1926. Notes on some Lepidoptera from eastern Texas. J. Agr. Res. 33: 
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Burns, J. M. 
1960. A new species of oak-eating Erynnis (Lepidoptera: Hesperiidae) 
restricted to the southern Cordillera of the United States. Was- 
mann J. Biol. 18: 147-160. 
1964. Evolution in skipper butterflies of the genus Erynnis. Univ. 
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