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During glacial periods of the Pleistocene, the climate in southeastern Arizona was much wetter and cooler. A woodland including pine, juniper and oaks formed a nearly continuous carpet across the region, filling what are now arid valleys between the mountains. In those periods, it is thought, populations of the jumping spider Habronattus pugillis were not so completely isolated as they are today.

As the climate grew drier and hotter, however, the woodlands retreated up the mountains, and the valleys were invaded by desert vegetation. The spiders retreated with the woodlands, and the populations on different mountain tops became isolated from each other.

Below: Four geographic forms of male H. pugillis and the mountain ranges in southern Arizona on which they occur. Depicted here are example forms from the Santa Catalina, Galiuro, Santa Rita, and Atascosa Mountains.

Copyright © 2002 National Academy of Sciences, U.S.A. All rights reserved. Click on image for larger (80kb) version.

Such a situation is ripe for speciation, as random genetic changes cause the isolated populations to become less and less like each other. However, Susan Masta and Wayne Maddison of the University of Arizona have found that the females of these populations are virtually indistinguishable. They haven't changed much over the years. But the males are very different on the different mountains.

If it were purely natural selection causing the populations to diverge from each other, it would affect both the males and the females. The fact that the males differ, but not the females, suggests that sexual selection is at play.

Reference: Masta, S.E. and W.P. Maddison. 2002. Sexual selection driving diversification in jumping spiders. Proc. Natl. Acad. Sci. U.S.A., Vol. 99, Issue 7, 4442-4447, April 2, 2002.