Iconic images of evolution – a fish wiggling up onto land, sprouting legs and hair, and standing up to become a man – are incredibly simplified in comparison to reality. Evolution is not a straight line so much as it is a tangled web of development, divergence, extinction and persistence. But scientists have often assumed that even on that tangled path, organisms do not usually throw their development in reverse.

But now, a genetic examination of house dust mites appears to have demonstrated that "reversible evolution" does, indeed, exist.

Some evolutionary biologists have argued that specialization is a irreversible process – that once an organism developed special features, it could not turn back and assume a form seen in its ancestral history. This hypothesis is called Dollo’s law, after Belgian paleontologist Louis Dollo.

Scientists have differed on just how rigid Dollo’s law could possibly be – whether it is merely an expression that it’s statistically very unlikely that an animal could follow the same evolutionary path twice (regardless of direction), or just an expression of how treading down one path leaves the path not followed, and all of its branches, closed off forever.

But the tiny house mite appears to have diverged from Dollo’s law, shedding the specialized parasitic lifestyle of its ancestors for a more generalized living feeding on skin flakes and other organic debris shed by humans.

"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," University of Michigan biologists Pavel Klimov and Barry O’Connor wrote in a paper published in the journal Systematic Biology on Friday.

There had been some debate in the house dust mite community over whether these little spider cousins evolved from a parasite or from another free-living organism. Klimov and O’Connor used large-scale DNA sequencing of more than 700 different mite species to evaluate 62 different hypotheses about dust mite evolution.

The family tree produced by the researchers’ team puts house dust mites as direct descendants of full-time parasites like the cat ear mite, and the psoroptic mange mites that afflict sheep and cattle.

The house dust mite’s evolutionary U-turn is particularly surprising, the researchers say.

"Parasites can quickly evolve highly sophisticated mechanisms for host exploitation and can lose their ability to function away from the host body," Klimov said in a statement on Friday. "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."

However, some of the characteristics of those parasitic ancestors could have aided house dust mites in their free-living states. The powerful digestive enzymes that allowed the parasitic mites to feed on their hosts help the house dust mite digest skin flakes and bits of feathers and nails that it finds lying around. Those digestive enzyme are also thought to be a big factor in why house dust mites cause so many allergies.

Another major factor in the house dust mite’s unconventional evolutionary path may lie in the fact that its ancestors frequently switched between different kinds of host animals. That variety likely kept the house dust mite’s ancestors from becoming too specialized to cope with life outside of a host system.

SOURCE: Klimov et al. “Is Permanent Parasitism Reversible? Critical Evidence from Early Evolution of House Dust Mites.” Systematic Biology 2013.

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