Evolution Bites the Dust

Article excerpt

Byline: Trevor Butterworth

Mites are regressing. Could humans do so, too?

Dust mites are known as tiny scavengers, living on the vast pall of dead skin shed by the animal world--and while they clean up the world in their own little way, they do so at the expense of triggering allergies in up to 1.2 billion people worldwide.

But the microscopic dust mite also turns out to be a peculiar example of evolution "in reverse"--or more precisely, despecialization--according to a new study by University of Michigan biologists Pavel Klimov and Barry OConnor. And this counters a long-held assumption in evolutionary biology known as Dollo's law (after the 19th-century Belgian paleontologist Louis Dollo), which stipulates that once you gain a complex trait, you can't return to the simpler states of your distant ancestors.

This poses a conundrum in the mite world. The distant ancestors of dust mites evolved to be able to feed directly on a bird or mammal--to become parasitic and therefore more complex. So how did the free-living dust mites get to be free living, and thus less complex? There were 62 different hypotheses in the academic literature trying to answer this question.

The 63rd looks like it will settle the issue. Klimov and OConnor embarked on an epic quest to sequence the DNA of 700 of the world's mite species chosen to produce a representative family tree. Sixty-four biologists from 19 countries collaborated. Finding one vital species of mite that happens to live only in the quill feathers of hummingbirds, took them, says Klimov, a month of fieldwork in Mexico.

But the result was clear. "The authors have made a compelling case that free-living dust mites evolved from a parasite, thus providing another counterexample to Dollo's law," says Jeff Gore, a biophysicist at the Massachusetts Institute of Technology who did groundbreaking work on bacterial evolution. …