There is new evidence suggesting that evolution does not proceed inexorably forward in a more or less straight line as many have long believed. Researchers have discovered that the common house dust mite has undergone “reverse” evolution, changing from a parasitic life form to a free-living one, presenting scientists with a new piece of the evolution puzzle. The theory of an organism reverting to the characteristics of its ancestors is known as “Dollo’s law.” The hypothesis, posed by Belgian paleonologist Louis Dollo, states that “an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.” Although evolutionary biologists have disagreed about how the rule is applied in nature, they have been in general agreement on the general principle that the evolutionary process is irreversible. Now biologists from the University of Michigan, relying on a large-scale genetic study of the house dust mite, seem to have discovered an example of reversible evolution, calling into question Dollo’s law. The study shows that the familiar house dust mite evolved over millions of years from free-living organisms into parasites before then evolving back into free-living organisms. “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 O’Connor of the University of Michigan Department of Ecology and Evolutionary Biology. Mites are members of the arachnid (spider) family. Known for causing allergic symptoms in humans, they are poorly understood from an evolutionary standpoint. According to Klimov and O’Connor, there are 62 different published hypotheses arguing about whether today’s free-living dust mites originated from a free-living ancestor or from a parasite. In an effort to paint a more comprehensive picture of the mites evolution, Klimov and O’Connor evaluated all 62 hypotheses. Using DNA sequencing, detailed evolutionary trees called phylogenies, and statistical analyses to test their hypotheses, the pair of researcher eventually recreated the evolutionary history of house dust mites. House dust mites appear within a lineage of parasitic mites, called Psoroptidia, which never leave the bodies of their hosts — mainly mammals and birds. The analysis shows that the immediate parasitic ancestors of house dust mites include skin mites, such as the mange mites of livestock and the dog and cat ear mite.
