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25Mar 2013

Dust Mite Stories Make “Reverse Evolution” Sound Like a Surprise. Is it Really?

Did these mites really devolve?

A handful of pieces that ran last week describing the first proof of “reverse evolution” were so confusing and odd that I had to send them to some biologists I know for a reality check.

What I found weird was that the pieces described the loss of a previously adaptive trait as some sort of shocker. In this latest case, scientists from the University of Michigan found that dust mites had gone from being parasitic to free-living, the change allegedly being surprising because the parasitic mites were thought to have evolved from a free-living ancestor.

The loss of a trait didn’t seems surprising to me, but maybe it was to biologists. It wasn't to the ones I consulted.    

Scientists have understood since Darwin that evolution is not an ascent up a ladder – it’s a process of adaptation to local environments (and some random drift.) The notion of “devolution” doesn’t make much sense in light of our modern understanding. And there are countless examples of species that gained and then lost adaptive traits - fur, flight, sight, and all kinds of other things.

A story by Trevor Butterworth from Newsweek ran in The Daily Beast  under the headline, Evolution Bites the Dust. The playful reference to dust mites might have made it forgivable had it been funny. (Well, I'm sure the creationists are laughing.)  Here’s how the importance of the mite study is described:

But the microscopic dust mite also turns out to be a peculiar example of evolution “in reverse”—or more precisely, despecialization—according to a new study by University of Michigan biologists Pavel Klimov and Barry OConnor. And this counters a long-held assumption in evolutionary biology known as Dollo’s law (after the 19th-century Belgian paleontologist Louis Dollo), which stipulates that once you gain a complex trait, you can’t return to the simpler states of your distant ancestors.

The paragraph and the rest of the piece conflated adaptation and complexity in a way that I found confusing. The scientists I called for a reality check assured me that I wasn’t missing something here. They do not have any such long-held assumptions about irreversibility in evolution, and Dollo’s law is hardly a cornerstone of their field. It’s so antiquated that they said they would be more surprised to find it was right.

The Daily Mail made the same point that Dollo's law is held sacred and that scientists believed evolution irreversible:

When it comes to evolution, there has always been the deep-rooted belief that you can't go backwards.

But new research has been published which challenges the idea that evolution is only moving forward in one direction. 

This piece in Redorbit also calls it a longstanding idea:

By studying dust mites, two University of Michigan biologists claim to have found evidence that contracts Dollo’s law – a long standing scientific belief claiming evolution is irreversible.

A piece in the New York Times by Douglas Quenqua also suggests that Dollo’s law is taken seriously by modern biologists. Well, it’s hard to make news out of a study that challenges a law most biologists don’t care about.   

The one piece on this issue of reversibility and evolution that impressed me was this story by Carl Zimmer. It ran in The New York Times in 2009. But that story described a study that reached the opposite conclusion – that evolution may really be irreversible. And to modern scientists, this new backing of an antiquated concept is truly surprising.

Zimmer describes the work of Joseph Thornton, who showed that when traits are lost, the molecular steps involved are not a reversal of the steps by which the trait was acquired. The organisms might have reverted to something similar to an ancestral phenotype but on a molecular level they were different. Zimmer seems to be the one with the right idea – The concept that evolution might be irreversible after all is what’s interesting and surprising.

Thornton says in the piece that he can’t prove reversal never happens, only that it’s very unlikely because so many evolutionary changes are built on a combination of multiple mutations, some of them neutral when acquired alone.

Zimmer's fascinating piece gives an example similar to the case of the dust mites – in which the acquisition of a trait was reversed. But the research profiled here is asking a deeper and more interesting question about whether the underlying molecular mechanism reversed:

In 2003, for example, a team of scientists studied wings on stick insects. They found that the insects’ common ancestor had wings, but some of its descendants lost them. Later, some of those flightless insects evolved wings again.

Yet this study did not necessarily refute Dollo’s law. The stick insects may indeed have evolved a new set of wings, but it is not clear whether this change appeared as reverse evolution at the molecular level. Did the insects go back to the exact original biochemistry for building wings, or find a new route, essentially evolving new proteins?

Also note that claims to the discovery of “reverse evolution” have cropped up here and again, as in this story about Seattle Fish and this one about a snouted chicken, though in this latter case genetic tinkering was involved. 

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