Irreversible Evolution? Dust Mites Show Parasites Can Violate Dollo's Law

A scanning electron microscope image of an American house dust mite. Our world is quite literally lousy with parasites. We are hosts to hundreds of them, and they are so common that in some ecosystems, the total mass of them can outweigh top predators by 20 fold. Even parasites have parasites. It's such a good strategy that over 40% of all known species are parasitic. They steal genes from their hosts, take over other animals' bodies, and generally screw with their hosts' heads. But there's one thing that we believed they couldn't do: stop being parasites. Once the genetic machinery set the lifestyle choice in motion, there's supposed to be no going back to living freely. Once a parasite, always a parasite. Unless you're a mite. In evolutionary biology, the notion of irreversibility is known as Dollo's Law after the Belgian paleontologist that first hypothesized it in 1893. He stated that once a lineage had lost or modified organs or structures, that they couldn't turn back the clock and un-evolve those changes. Or, as he put it, "an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors." While some animals seem to challenge Dollo's Law, it has long been a deeply held belief in the field of parasitology. Parasitism is, in general, a process of reduction. Adjusting to survival on or in another animal is a severe evolutionary undertaking, and many parasites lose entire organs or even body systems, becoming entirely dependent on their hosts to perform biological tasks like breaking down food or locomotion. Parasitology textbooks often talk about the irreversibility of becoming a parasite in very finite terms. "Parasites as a whole are worthy examples of the inexorable march of evolution into blind alleys" says Noble & Noble's 1976 Parasitology: the Biology of Animal Parasites. Robert Poulin is even more direct: "Once they are dependent on the host there is no going back. In other words, early specialisation for a parasitic life commits a lineage forever." Now, parasites are proving that not only can they evade immune systems, trick other animals, and use their hosts' bodies in hundreds of nefarious ways, some can go back to living on their own. This is exactly what scientists now believed happened in the Pyroglyphidae — the dust mites. Mites, as a whole, are a frighteningly successful if often overlooked group of organisms. More than 48,000 species have been described. These minuscule relatives of spiders can be found worldwide in just about every habitat you can imagine. Many are free-living, but there are also a number of parasitic species, including all-too-familiar pests like