BARRIERS TO GENE FLOW IN NATURAL 
POPULATIONS OF GRASSHOPPERS 
I. THE BLACK CANYON OF THE GUNNISON RIVER 
AND ARP HI A CONSPERSA . 1 
By Robert B. Willey and Ruth L. Willey 2 
Introduction. 
The oedipodine grasshopper, Arphia conspersa , is polymorphic for 
wing color in Colorado. This feature may prove useful as a marker 
for tracing historical patterns of gene flow between populations. The 
metathoracic wings vary from Vermillion through salmon-orange to 
yellow with several intermediate shades which are almost certainly 
heterozygous in genotype. Although the known populations of the 
Colorado Front Range are all polychromatic (Gordon Alexander, 
personal communcation) , populations in the Gunnison River Basin 
on the Western Slope often are monochromatic. For example, those 
to the south in the foothills of the San Juan Mountains are uni- 
formly yellow as a rule, and those to the north in the Elk and West 
Elk Mountains are usually salmon-orange. However, near the Gun- 
nison River, which runs west through the center of the basin, there 
occurs a complex series of populations in various stages of introgression 
and separation. We wish first to present the case of the Black 
Canyon of the Gunnison River which presents a striking example 
of a topographic barrier to gene flow, reminiscent of the cases of the 
Abert-Kaibab squirrels and other mammals on the north and, south 
rims of the Grand Canyon in Arizona (Allee, et al 1949, p. 608; 
Hall and Kelson, 1959). 
The Black Canyon. The canyon is located in the west central 
part of Colorado, about 60 miles southeast of Grand Junction. It is 
about 50 miles long, 20 miles of which are included in the Black 
Canyon National Monument. In forming this deep canyon, the 
Gunnison River has worn its way vertically through about 2000 feet 
of hard pre-Cambrian rocks, primarily gneiss and granite. The river 
Research conducted at the Rocky Mountain Biological Laboratory, 
Crested Butte, Colorado. 
Address: Department of Biological Sciences, University of Illinois at 
Chicago Circle, Chicago, 60680. 
Manuscript received by the editor December 31, 1966. 
Explanation of Plate 5 
Southeastward view of the North and South Rims of the Black Canyon 
from Fruitland Mesa. Grizzly Gulch (locality #23) is the valley in the 
foreground, Coffee Pot Hill and Poverty Mesa are in the upper left back- 
ground. 
42 
