60 
Psyche 
[March 
Pogonomyrmex ant nests in the developing Pliocene Sonoran 
desert, became isolated and, under selective pressures applied by 
ants, differentiated into Echinocoleus. 
4. This early Echinocoleus extended its range eastward into 
higher elevation grassland and woodland habitats and eventually 
into the Rio Grande River drainage and the developing Chihuahuan 
desert. The lack of similarity in the vegetation of the two deserts 
show they have been isolated by intervening regions of forest, wood- 
land, or grassland since the start of Tertiary regional aridity. 
5. The ancestral Echinocoleus separated into a lower elevation 
sonorensis ancestor and a higher elevation setiger-chihuahuensis 
ancestor, and the first proceeded to differentiate more rapidly than 
the second, probably through ant-applied selective pressures. 
6. The separation of the woodland-grassland range of setiger- 
chihuahuensis into an eastern (Texas-New Mexico) chihuahuensis 
ancestor and a western (Arizona) setiger ancestor occurred during 
a Pleistocene glacial when extensive conifer forests occupied the 
north-south trending mountainous region of eastern Arizona and 
western New Mexico (Martin and Mehringer, 1965). The forest 
extended south to the Sierre Madre Occidental at the western edge 
of the Mexican Plateau. In at least the last glacial this forested bar- 
rier was unsuited for Pogonomyrmex and thus for Echinocoleus, 
and it was probably similarly unsuited in earlier glacials. During 
these glacials, the Sonoran desert was diminished in size and pushed 
closer to the Gulf of California, and the Chihuahuan desert was 
reduced to lower elevation regions of the Rio Grande drainage 
(Wells, 1966). This greater separation of arid Sonoran and Chi- 
huahuan regions is known to have caused range separation, diver- 
gence, and formation of species pairs in many groups of arid-land 
insects (Cohn, 1965; Howden, 1969). 
7. The western setiger ancestor shifted, into an occupation of 
Novomessor ant nests. 
8. The three isolated stocks of Echinocoleus then differentiated 
into the three present species. 
9. Since the last glacial maximum, some of the species of Pogono- 
myrmex have expanded from either or both of their contracted 
Sonoran or Chihuahuan refugia to their presently continuous and 
overlapping ranges, covering most of the southwestern US. Echi- 
nocoleus came to occupy their present non-overlapping ranges 
following the ants’ readjustments to the present, post-glacial climatic- 
vegetational conditions. 
