And as demonstrated among chubs of the genus Gila in the Colorado river
basin, hybridization is one way new species can be formed. Virgin River
roundtail chub (Gila seminuda) arose as a hybrid between G. robusta and G.
elegans. Pahranagat roundtail chub (G. jordani) is believed to be hybrid
between G. robusta and humpback chub (G. cypha). Headwater chub (G. nigra)
is hypothesized to be of hybrid origin between G. robusta and Gila chub (G.
intermedia). How these hybridizations have occurred is unknown, although
scientists speculate that extremely arid conditions could have been a major
factor, as illustrated in the following scenario.
Two closely related species occur together in a desert stream. As conditions
become more arid the river decreases in size, which potentially causes the
populations of both species to decrease as well. Sex ratios may also be
affected so that fish from one species have to look to the other species for
a mate. The species interbreed, then the hybrids breed, and a hybrid
population is formed. Since hybridization produces genetic changes faster
than the usual course of mutation over time, hybrid stocks may have had a
head start in responding to the ever-increasing aridity of the American
Southwest. In other words, hybridization may have been advantageous in Gila
as a selective response to rapidly changing environmental pressures.
Chris Scharpf
Baltimore
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