56 
Psyche 
[March 
They are equivalent in elevation, climate and biotic community 
structure, and are separated by no more than one mile of horizontal 
space. Nor does the wing-color of A. conspersa appear to be influ- 
enced by external factors such as humidity, extremes of temperature, 
etc. Since homogeneous populations of both colors are found in 
equivalent habitat and climatic extremes elsewhere, ontogenetic and 
selective hypotheses seem unlikely, although untested experimentally. 
The wings are kept folded during courtship and the female has no 
opportunity to see the color of the male’s wings; therefore, sexual 
selection is not likely during the ground interaction, at least. There 
is, of course, the possibility that color is accompanied by some other 
trait for wing color pleiotropically determined genetically with some 
behavioral effects which could have sexual selective value. 
As hypothesis alternative to one which assumes selective differences 
in the two areas supposes that these populations represent partially 
arrested introgression of two formerly distinct geographic variants 
which reached the Black Canyon after it had acquired much its 
present form. It is not yet clear to us what might have caused the 
homogeneity and isolation of the variant populations — if a poly- 
morphic species was indeed the progenitor. However, our mapping 
of the populations throughout the Gunnison Basin has shown a large 
orange-winged population centered near Grand Mesa and the Elk 
Mountains and a homogeneous yellow-winged population centered 
in the San Juan Mountains. The analysis of the zoogeography of 
this species in the Gunnison Basin soon will be completed. Perhaps 
the heavy Pleistocene glaciation of the area (Richmond, 1965) may 
hold a clue to the origin of these populations, but such slow speed of 
gene flow since then may well be due to the slow rate of dispersal — 
the low vagility of the species due to the combination of social attrac- 
tion and relatively restricted habitat preference. 
Whatever the origin of the homogeneous populations, the barrier 
of the Black Canyon of the Guninson River evidently acts as a fine- 
meshed filter with discontinuous habitats scattered infrequently over 
a spectacularly difficult terrain. The “insularized” populations may 
also reach their reproductive peaks at different times due to the 
seasonal effect of altitude on their phenology. Barring undetected 
selection factors such as predator preference for the rare variant, a 
filter effect could work multidimensionally to prevent thorough intro- 
gression of these two variants. 
Summary 
The Black Canyon of the Gunnison River in Colorado may be a 
nearly complete barrier to direct gene flow between populations of 
the oedipodine grasshopper, Arphia conspersa. In this species, a con- 
