BARRIERS TO GENE FLOW 
IN NATURAL POPULATIONS OF GRASSHOPPERS 
II. MAINTENANCE OF NARROW HYBRID-ZONES 
BETWEEN MORPHS OF ARPHIA CONSPERSA 
ON BLACK MESA, COLORADO 1 
By Robert B. Willey and Ruth L. Willey 2 
In the previous publication of this series (Willey and Willey, 
1967), we described the zoogeography of Arphia conspersa on the 
two sides of the Black Canyon of the Gunnison River in southwestern 
Colorado. North of the canyon the populations are monochromatic 
for red-orange wing color and south of the canyon the populations 
show a steep cline from west to east culminating in nearly 100% 
yellow-winged demes even though they often are less than one aerial 
mile from the orange-winged populations on the North Rim. We 
concluded that the sides of the canyon were an effective barrier to 
gene flow between the two rim populations and that the sedentary 
behavior and social cohesiveness of the adults (Willey and Willey, 
1967 and 1969) probably slowed down lateral gene diffusion between 
the nearly adjacent demes of the South Rim. 
We have completed a preliminary survey of a narrow hybrid-zone 
on Black Mesa, an adjacent area which we previously have discussed 
briefly (Willey and Willey, 1967). The barriers seem to be related 
to suitable habitat and, in this case, we hope to show that the mixed 
demes are subject to periodic extermination, and perhaps are main- 
tained as a hybrid-zone by reinvasion from the neighboring mono- 
chromatic populations. 
Methods 
The census method is that of Willey and Willey (1967). Briefly, 
we walked in a non-repeating spatial pattern through the habitat 
and scored each insect as it flew up as either orange or yellow. An 
effort was made to count 100 individuals in each contiguous deme 
(200 preferably), but counts as low as 10 are reported in the protocol 
(Fig. 1 and 2). Nearly all the populations on the mesa were cen- 
sused at least once during the six years of study; several were cen- 
sused as many as ten times. If we felt the population could withstand 
Research was based at the Rocky Mountain Biological Laboratory, 
Crested Butte, Colorado; and X Lazy F Ranch, Crawford, Colorado. 
Address of the co-authors: Department of Biological Sciences, University 
of Illinois at Chicago Circle, Chicago 60680. 
Manuscript received by the editor January 18, 1972 
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