Death to Dollo’s Law!

Hint for science journalists: if the hook to get readers to pay attention to your story is to warn them to sit down because a 19th century “law” of evolution has been shown to be wrong, you’re going to irritate scientists, who will then write rude blog posts sneering at your writing. That’s the case with this story titled The Wrists of Birds Reveal Evolution Undoing Itself, which is also subtitled Contrary to earlier claims, a new study shows that evolution may be reversible.

It’s describing a paper on the re-emergence of a wrist bone in birds, and it touts Dollo’s Law.

The 19th-century biologist Louis Dollo taught that evolution is irreversible; once a structure is lost, that pathway is closed forever. It’s a principle now known as Dollo’s law.

It was called a law; it got bandied about in the literature extensively; and then, as we acquired a greater understanding of genes and molecules and the patterns of change in evolution, it was rendered obsolete and more or less wrong. What does it even mean to say that a single nucleotide change, for instance, has a “direction” or is irreversible? What we should be recognizing is that many morphological changes are a consequence of a whole series of accumulated mutations, and it’s extremely unlikely that that a series of random changes would neatly reverse all of them.

I have a longish rant about Dollo’s Law and it’s continued promotion by biologists. I know that a key to getting your story heard is declaring that it breaks expectations, but really, people, breaking a dead, 125 year old “law” isn’t news any more.

And it’s a shame, because it’s an interesting result. A wrist bone, the pisiform, disappeared in dinosaurs that had a relatively simpler forelimb; in their descendants, the birds, a new wrist bone appeared, called the ulnare. The research argues that the ulnare is the pisiform. Which wouldn’t surprise me at all. But then a scientist has to go and say something stupid to the press.

But in analyzing the development of the ulnare, Vargas showed that it is, in fact, the re-emergence of the pisiform. “While the physical expression of a gene may be suppressed, it doesn’t mean the possibility of generating that structure has disappeared,” says Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County. The gene is still there, it’s just dormant.

That last sentence? Stupid. No, the “gene” wasn’t dormant. First of all, there is no “pisiform gene” — there is a whole set of interacting genes that regulate how condensing bones in the wrist establish themselves and differentiate, and those genes are sensitive to the environment, both in the constraints on structure and in their epistatic relationship with other genes. Those genes were almost certainly functional in their entirety in birds and dinosaurs, but the dinosaur forelimb was an environment that did not promote the formation of a specific bone called the pisiform. Changes that promoted larger shape changes in the limb produced conditions under which those same genes would re-express themselves in a way that led to the formation of that bone.

Genes do not lie dormant for a hundred million years. Genes that do nothing accumulate mutations that are not removed by negative selection, and are reduced to pseudogenes, and eventually, after a very long time, are shredded by deletions and are lost.

Scientists who are not hobbled by an obsolete understanding of the evolutionary process that is not just pre-molecular biology, but pre-genetics, understand this.


  1. Al Dente says

    But…but…but…Dollo’s Law is a LAW! As all the best creationists tell us, a law is higher than a mere theory. Just like the Second Law of Thermos Bottles invalidates the theory of evolution, Dollo’s Law doesn’t allow evolution to grow the same bone in dinosaurs and birds. Besides, the name Dollo is ubercool so the law gets reflected coolness which makes it impermeable or something.

  2. azhael says

    I really hate how biology is butchered by journalists…they are so keen to abuse anything they can possibly transform into a sensationalistic monster that any scientific content is disfigured and lost. This is a real problem that is causing harm, it’s not just a personal pet-peeve. The misrepresentation of science and the surreal bollocks that they invent to always create a sensationalistic (and fictional) spin, does actually cause harm.

  3. slatham says

    Perhaps the wikipedia entry on atavism could use an edit? Currently: “Evolutionarily, traits that have disappeared phenotypically do not necessarily disappear from an organism’s DNA. The gene sequence often remains, but is inactive. Such an unused gene may remain in the genome for many generations.[4] As long as the gene remains intact, a fault in the genetic control suppressing the gene can lead to it being expressed again. Sometimes, the expression of dormant genes can be induced by artificial stimulation.”

  4. says

    I really hate how biology is butchered by journalists

    It’s not just journalists, and often not journalists’ fault at all. In the story above, the problem is with the quoted statement from the director of a museum, a PhD paleontologist. Should the journalist be expected to counter this experienced scientist’s statement, or just say, “that’s BS; I’m not printing it”? Also, you can’t simply blame press releases for a lot of this. In my experience working with a university PR department putting out press releases about my late wife’s work, they always deferred to us about what was going to be said; we had the final say. Face it, scientists often push the “groundbreaking”, “overturning”, “the ___ has to be completely redrawn” stuff; would you, should you, expect the PR department, or a journalist, to be the ones to counter them with a dose of reality?

  5. marcus says

    This just in! In an apparent reversal of Dollo’s Law flightless birds have been discovered who were once able to fly! Evidently the “non-flight” gene’s effect has re-emerged. “The gene for non-flight has always been there, it was just dormant.”

  6. Amphiox says

    Recalling that recent thread about theories, hypotheses and laws in science, I note that Dollo’s Law was never even actually a law. It just called one by social convention among the scientists of the time.

  7. chrislawson says

    azhael@3: Yes, journalists often butcher science, but this appears to be an example of a journalist directly quoting a scientist being wrong. It happens. The fault here lies with Dr Chiappe, who should know better as his area of expertise is evolution of the archosaurs. The only way to fault the journalist here is if Dr Chiappe was misquoted.

  8. iknklast says

    Chris Lawson:

    The only way to fault the journalist here is if Dr Chiappe was misquoted.

    Which of course happens. I was quoted extensively in our local newspaper last year. The only problem is, I didn’t actually say any of the words that were in the article, and in fact, some of the words I was quoted as saying I’ve never said in my life (except in the letter to the editor to point out the problem). The journalist made up everything to fit what she thought I should be saying, never mind what I did say because she didn’t write any of it down, and didn’t carry a recorder.

    That doesn’t mean Dr. Chiappe was misquoted. But I’m willing to consider that as a strong possibility.

  9. Menyambal says

    Yeah, I just watched some TV preacher quoting Antony Flew, that very influential atheist who had turned to Jesus because of DNA. I had to go look up Antony Flew, because he’d never influenced me, and he certainly never turned to Jesus – a sort of Aristotlean deism, maybe.

  10. marcoli says

    The wrist bone in dinosaurs was not necessarily ‘lost’ in the first place. Bones start out as cartilage elements in an embryo, and then cartilage tissue is replaced by bone. So what could have happened in dinos was that the cartilage element stopped converting to bone for a while, resulting in the ‘loss’ of the bone in that lineage in dino skeletons. Then in birds the cartilage element was selected to become bone. Taken that way the whole thing is not a big deal at all.

  11. David Marjanović says

    Hey, PZ, great that you blog about this, but why don’t you link to the paper? It’s in PLOS Biology, an open-access journal! And the figures are stunning.

    in their descendants, the birds, a new wrist bone appeared, called the ulnare

    No; a new wrist bone appeared which was misidentified as the ulnare, a basic wrist bone that is common to all… sarcopterygians or so that haven’t lost it. You have it, and so do the coelacanths. The surprise here is that the new bone isn’t the reappeared ulnare, but the reappeared pisiform; the real ulnare forms and then disappears in the embryo. The oldest dinosaurs had ulnaria and pisiforms, and both were lost in the earliest tetanuran theropods some 200 million years ago.

    Bones start out as cartilage elements in an embryo

    Not all do.

  12. raven says

    …and he certainly never turned to Jesus – a sort of Aristotlean deism, maybe.

    That was one of the more disgusting things the fundia xians have done.

    Antony Flew was very old and suffering from an age related cognitive condition. The xians used him as a meat puppet. Shortly after discovering Deism, he died of old age. It’s not even clear that most of what he wrote, he actually wrote. It’s likely it was ghost written by predatory and dishonest xian liars.

    And Antony Flew who? Most atheists had never heard of him until the meat puppet incident.

  13. moarscienceplz says


    Should the journalist be expected to counter this experienced scientist’s statement, or just say, “that’s BS; I’m not printing it”?

    I would expect any journalist covering a science story to understand that there is almost never a single gene involved in a particular phenotype feature, and to ask the scientist if she/he wants to restate such an obviously wrong sentence. If the scientist then stands his/her ground, then I would expect the journalist to seek a different expert source to point out the flaw in the original quote, or not use it at all.
    Repeating false information just because it came from some supposed expert is cowardly and wrong.

  14. David Marjanović says

    I would expect any journalist covering a science story to understand

    Well, that’s it: science journalists generally have no idea what they’re writing about. The very few exceptions quickly become famous, actually.