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The Modest House Dust Mite Helps Topple a Darwinian Evolutionary Postulate

scabies mite.jpg

Mites are on our family's mind. Our kids' school is embroiled in a semi-hysterical mini-scare over a possible outbreak of scabies, an itchy rash associated with scabies or "itch" mites (Sarcoptes scabiei). The condition is highly contagious in a school or similar institutional setting, though we've learned that diagnosing the presence of these mites is more complicated than you might think. Meanwhile our daughter, we'll call her N, comes home from school with tales of bugs emitting from fans and heaters. But scabies mites, we point out to her, are microscopic arthropods (pictured above).

Scabies mites are also parasitic, meaning that they live in a permanent relationship with a living host, not a fan or a heater. One way you get rid of them is by putting clothing, bedding and other personal articles belonging to the afflicted person in plastic bags for some number of days. Deprived of access to the host, the bugs die. Other mites are free-living, however, meaning that they are not directly dependent on a host but can scrounge for sloughed-off organic nourishment in an animal's nest or a human dwelling.

With all this going on, I found it interesting that mites are also in the news, even ascending to the top stories of the day as evaluated by Drudge. The common house dust mite, in fact, has helped topple an evolutionary postulate: Dollo's law, which holds that evolutionary development should be unidirectional. Michael Behe explained here a while back:

Louis Dollo, an early 20th-century paleobiologist, was interested in discerning phylogenies. He maintained that one could always distinguish ancestral forms from descendant forms. Stephen Jay Gould (1970) commented that Dollo's "law" was not an empirical observation, but rather a postulate which he felt was necessary to properly construct phylogenies. Over the years the meaning of "Dollo's law" transmogrified. In modern usage, the phrase has come to mean that complex traits, once lost, do not re-evolve in the same lineage. For example, whales do not re-evolve gills, even though they are aquatic creatures who descended from fish, because gills are a lost, complex trait in that lineage.

Dollo's law is taken with a grain of salt by many biologists, and apparent exceptions to the law have been noted (cited in Bridgham et al 2009).

It makes sense that Darwinian evolution should be irreversible. (See Casey's comments.) If every mutational step, for example, from a free-living lifestyle to a highly specialized parasitism offered some advantage to a kind of organism, as the classic Darwinian model assumes, then backing up should be difficult or impossible. That way, every step in reverse would mean giving up an advantage. In Darwinian evolutionary terms, where only helpful or neutral changes are retained, that shouldn't happen very often if at all. Of course, in a model that includes room for design input, it could.

So there comes along a new study in Systematic Biology by two University of Michigan researchers that seems to definitively show how evolution in reverse is exactly what happened with free-living dust mites. These bugs, a very common source of allergies, make their home in your carpet, sofas, and the like, no matter how scrupulous you are about housecleaning. You can go here and see a picture of one under an electron microscope. They're not very attractive.

More to the point, they evidently are the descendants of parasites that in turn, over the course of millions of years, descended from free-living earlier ancestors. This is what the Michigan team shows. "Is Permanent Parasitism Reversible?", Pavel Klimov and Barry OConnor ask in the title of their article. It's a question that, in a different context, I imagine many parents of adult children who are still living at home may ask from time to time. On the basis of genetic analysis, Klimov and OConnor place dust mites (belonging to the family Pyroglyphidae) in the same suborder to which scabies mites, our good friends, belong.

So we come full circle. They conclude:

Our analyses recovered pyroglyphids deeply nested within a large monophyletic lineage otherwise comprising parasites of mammals and birds -- an unranked clade termed Psoroptidia.
Free-living mites in a parasitic lineage. How did they get there? More:
Results from our phylogenetic analyses, topology-based tests for alternative placement of house dust mites, ancestral character state reconstructions, and a test for irreversibility of parasitism all suggest that the common ancestor of pyroglyphid house dust mites underwent reversal from a permanently parasitic lifestyle to become secondarily free living. By inference, this violates the ecological interpretation of Dollo's law defining the parasitic state as irreversible because many of the adaptations necessary for life away from the host ("complex state") are assumed to be lost in parasites (Cruickshank and Paterson 2006). Thus, as exemplified by house dust mites, specialized organisms can adapt to new ecological niches via de-specialization, escaping evolutionary "dead ends."
From the perspective of orthodox Darwinism, whose logic seems to demand a more or less rigid application of Dollo's law, might this be considered an awkward conclusion? It mite.

UPDATE: I just received this email from my wife: "Dr. G looked at N, we told her the whole story, and she concluded that she doesn't have scabies and never did. Yay!!!!" So it just goes to show.

Image: Sarcoptes scabiei, Wikipedia.