House Dust Mite Study Shows Reverse Evolution Possible

A new study shows that certain organisms can undergo reverse evolution, that is, go back to the way they were living before evolving new traits. The study challenges the idea that evolution is uni-directional and an organism can't undo changes.

The idea that organisms can't revert back is held by Dollo's law, which states "that evolution is not reversible; i.e., structures or functions discarded during the course of evolution do not reappear in a given line of organisms. The hypothesis was first advanced by a historian, Edgar Quinet."

The present study was conducted by a research team led by two biologists from University of Michigan who conducted the study on common house mites that live on mattresses, sofas and carpets. These mites have evolved from parasites that have in turn come from free-living microbes.

"All our analyses conclusively demonstrated that house dust mites have abandoned a parasitic lifestyle, secondarily becoming free-living, and then speciated in several habitats, including human habitations," according to Pavel Klimov and Barry OConnor of the U-M Department of Ecology and Evolutionary Biology.

Researchers found that the mites have gone back to the ways of their ancestors by beginning to live independently; a phenomenon which goes against the idea that evolution only goes forward.

Researchers, for the study, looked at 62 hypotheses on the subject. They then conducted extensive DNA analyses on more than 700 species of mites, to create a kind of family tree (phylogenetic tree) for the mites. Complex statistical analyses were conducted to test each hypothesis that explains the ancestry of mites.

The phylogenetic tree of the mites showed that the mites' close relatives are the Psoroptidia, which are full-time residents in the bodies of birds and animals. Also, the immediate ancestor of the mice in carpets and mattresses are the mites that live in the fur of dogs and cats.

"This result was so surprising that we decided to contact our colleagues to obtain their feedback prior to sending these data for publication," said Klimov, the first author of the paper, according to a news release.

Klimov and OConnor say that many changes made mites free-living, which was significantly different from their parasitic past. Few traits that contributed to this reverse evolution were: the ability to withstand an environment that has low humidity, a powerful digestive system that allowed them to feed on skin and hair, and their ability to switch from one host to another.

Once humans began living in homes, the mites moved in too. And, their ability to trigger immune response began creating allergies in humans.

"Parasites can quickly evolve highly sophisticated mechanisms for host exploitation and can lose their ability to function away from the host body. They often experience degradation or loss of many genes because their functions are no longer required in a rich environment where hosts provide both living space and nutrients. Many researchers in the field perceive such specialization as evolutionarily irreversible," Klimov said.

The study is published in the journal Systematic Biology.