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Single gene flip, not Darwinism, explains butterfly mimicry but it’s not clear why the butterflies bother

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How a 'flipped' gene helped butterflies evolve mimicry
swallowtail mimics and polymorphs/Matt Woods, UChicago

From Phys.org:

Female swallowtail butterflies do something a lot of butterflies do to survive: they mimic wing patterns, shapes and colors of other species that are toxic to predators. Some – but not all – swallowtail species have evolved several different forms of this trait. But what kind of genetic changes led to these various disguises, and why would some species maintain an undisguised form when mimicry provides an obvious evolutionary advantage?

In a new study published this week in Nature Communications, scientists from the University of Chicago analyze genetic data from a group of swallowtail species to find out when and how mimicry first evolved, and what has been driving those changes since then. It’s a story that started around two million years ago, but instead of steady, progressive changes, one chance genetic switch helped create the first swallowtail mimics. And it has stuck around ever since.

If it is not progressive change, it is not Darwinism

For a long time, scientists thought that butterfly mimicry was controlled by “supergenes,” groups of several tightly linked genes that were always inherited as a group. In a 2014 study, Kronforst and his colleagues showed what appears to be a supergene is actually a single gene called doublesex that controls the different color patterns and shapes we see in female swallowtails.

It’s not at all clear whether double sex is an advantage to the butterfly:

The researchers also looked at what maintained polymorphism over time. One cause could be sexual selection, that males prefer certain female color patterns over another. Previous research on mating behavior doesn’t back up that idea though. Another possibility is “crypsis,” or the idea that undisguised females blend into their natural surroundings better than the mimics. Kronforst and the team tested that hypothesis by comparing mimetic and non-mimetic females against a green forest background using models for predator (i.e. bird) vision. The non-mimetic, undisguised females actually don’t blend in to the background any more than mimics, so this idea is out too

Those two findings, combined with the genomic sequence data, led the researchers to start thinking about another intriguing possibility. It could be that the genetic changes that led to mimicry in the first place also built in some long-term disadvantages. When the original doublesex gene inverted, it probably carried a bunch of other unrelated genetic material with it. Since the flipped doublesex gene can’t be recombined with its original version, the extra stuff has “hitchhiked” ever since—and it could have consequences. In fact, some research shows that female mimics don’t live as long as standard ones.

“We think a bunch of differences were accidentally captured when one copy of the gene flipped and became the mimetic copy. Because a lot of those changes are functional, they could be detrimental to health,” Kronforst said.

“The idea is that you have this hardwired disadvantage to mimicry. The standard females don’t have the protection of mimicry, but they also don’t have this inherent genetic cost and these two things offset one another” he said. More.

It’s all fascinating. The big disadvantage of Darwinthink, of course, is that it can explain everything and anything and therefore, eventually, nothing. A theory that could make highly specific predictions might be more useful. One learns more by confuting specifics than by cming up with ad hoc explanations.

See also: Convergent evolution: Speciation in butterflies an unusually tough mess

and

Evolutionary convergence of butterflies

The traditional “supergene” view:

The caterpillars have their own tricks:

4 Replies to “Single gene flip, not Darwinism, explains butterfly mimicry but it’s not clear why the butterflies bother

  1. 1
    polistra says:

    I bumped into an equally illogical case of Darwinian blindness while studying to get details right for courseware.

    From a Wikipedia article on the cochlea’s automatic gain control system:

    The strength of the reflex is weakest for pure tones, and becomes stronger as the bandwidth of the sound is increased. … Animals with the strongest MOC reflex sustain less hearing damage to loud sounds. This proposed biological role of the MOCS, protection from loud sounds, was challenged by Kirk and Smith (2003), who argued that the intensity of sounds used in the experiments (?105 dB SPL) would rarely or never occur in nature, and therefore a protective mechanism for sounds of such intensities could not have evolved.

    ==========

    Kirk and Smith believe the system couldn’t really exist because Kirk and Smith can’t imagine a noisy Savannah.

    A proper Darwin syllogism would go like this:

    1. Experiments show that the feedback mechanism kicks in at 105 dB.

    2. It must have evolved to increase fitness.

    3. Therefore we need to find a situation where loud sounds make survival difficult.

    In fact this situation exists in EVERY ANIMAL that has a larynx. When a human or a Sabertooth Tiger makes a sound, the level INSIDE ITS OWN SKULL easily exceeds 105 dB, so the cochlea and the sound-reading software need compensation.

    If Kirk and Smith had been using Darwin logic, they would have found this situation. But they weren’t. They were just listening to TED Talks.

  2. 2
    mike1962 says:

    The cookie expires, what, 7 days? Stop it.

    P.S. Disqus would be a far better blog situation. Try it. Believe me. It would be better.

    Other than that, good show

  3. 3

    mike1962 @ 2: Pardon my ignorance. What exactly are you talking about?

  4. 4
    awstar says:

    It’s a story that started around two million years ago,

    FAKE science alert. It would be more accurate to say the story started less than a couple of hundred years ago, and has led us further and further away from the truth ever since. But if we never call out these obvious misrepresentations that they assert to be a fact — no matter how minor or irrelevent they seem, we’ll never get back to where immutable truth lives.

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