Hugh Pickens writes writes "In evolutionary biology, there is a deeply rooted hypothesis called Dollo's law that evolution is unidirectional and irreversible and that once an organism has evolved specialized traits, it can't return to the lifestyle of its ancestors. According to this hypothesis a structure or organ that has been lost or discarded through the process of evolution will not reappear in exactly the same form in that line of organisms. Now Science Daily News reports that researchers have used a large-scale genetic study of the lowly house dust mite to uncover an example of reversible evolution that appears to violate Dollo's law. The study shows that tiny free-living house dust mites evolved from parasites, which in turn evolved from free-living organisms millions of years ago. "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," say Pavel Klimov and Barry OConnor of the U-M Department of Ecology and Evolutionary Biology. The project used large-scale DNA sequencing, the construction of detailed evolutionary trees called phylogenies, and sophisticated statistical analyses to test the hypotheses about the ancestral ecology of house dust mites. The result was so surprising largely because it runs counter to the entrenched idea that highly specialized parasites cannot return to the free-living lifestyle of their ancestors. "Parasites can quickly evolve highly sophisticated mechanisms for host exploitation and can lose their ability to function away from the host body," says Klimov. "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.""