Stolen glance

Charles Darwin eloquently expressed his own struggle with imagining how the vertebrate eye could have evolved.  If you spend any time reading the writings of creationists or proponents of intelligent design (not recommended unless you have an extraordinary tolerance for pretzel logic), you'll find a quote from The Origin of Species:

To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.

This quote causeth much crowing and fist-bumping amongst the holy, lo unto this very day, usually followed by something like "Even Darwin admitted that evolution by natural selection doesn't work."

It's wryly amusing, given the degree to which anti-evolutionists cherry-pick the scientific evidence they accept and the (much larger amount of) evidence they ignore completely, that this quote is itself cherry-picked, as you'd find out if you went on to read the next two sentences of the book:

When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science.  Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory.

So the argument -- if I can dignify it by that name -- of the anti-evolutionists boils down to our old friend Argument from Incredulity: "I can't imagine how it could have happened, therefore it must be God."

The truth is, we understand the evolution of the eye pretty well.  Lots of animals (for example, flatworms) have light-sensitive spots; and as Richard Dawkins brilliantly explains in his tour-de-force defense of evolution The Blind Watchmaker, once you have any kind of light-sensing ability at all, incremental improvements can result in some amazingly complex structures.  The eye isn't "irreducibly complex" -- the intelligent design cadre's favorite phrase -- at all; simple photosensitive spots led to "cup eyes" which led to eyes like a pinhole camera, and so on.  In fact, the whole process has been repeated more than once.  Complex eyes have evolved independently at least three times, possibly more.

[Image licensed under the Creative Commons Kamil Saitov, Human eye iris 5, CC BY 4.0]

The vertebrate eye is a particularly interesting case.  The transparent proteins in the lens, appropriately named crystallins, were found in 1988 by molecular biologist Joram Piatigorsky to come from the same genes that produce heat-shock proteins, enzymes that protect other proteins against damage from fluctuating temperature.  Take heat-shock proteins and assemble them in layers, you get a lens.  This is an example of exaptation (also called preaptation or preadaptation), where a gene, protein, or structure that evolved in one context develops a function giving it an entirely different use, and that use kind of moves in and takes over.

It's another example of exaptation in the eye that is why the whole topic comes up; in fact, it's not only exaptation, it's exaptation of a gene that was borrowed from another organism entirely.  A paper this week in Proceedings of the National Academy of Sciences looked at a protein in all vertebrate eyes called IRBP (interphotoreceptor retinoid-binding protein), without which our sense of sight wouldn't work.  When light strikes your eye, protein-bound complexes containing retinol (a derivative of vitamin A) absorb the energy, causing them to kink.  This triggers a neuron to fire, sending a signal to your brain.  However, something needs to unkink the complex, thus resetting the switch so it can respond to the next photon to come along.

That's what IRBP does.  Without it, your retinal cells would be able to respond exactly once, then they'd shut down permanently.

This week's paper found something astonishing.  The gene that codes for IRBP doesn't exist in our nearest invertebrate relatives, nor in any other group studied, with one exception -- certain species of bacteria.  What apparently happened is that the common ancestor of all vertebrates swiped a gene from bacteria that coded for a pepsidase -- an enzyme that breaks down and recycles proteins.  This kind of gene-stealing isn't uncommon.  (I did a post a few years ago about a pair of viral genes that seem to be critical for our forming memories, if you want another good example of this phenomenon.)  But like the heat-shock proteins becoming crystallins, the pepsidase made by the gene our ancestors grabbed was useful for something else -- unkinking the protein complexes in our rapidly-evolving eyes.

So our eyes work not only because of proteins gaining additional functions, but because we stole a gene from bacteria.

"Horizontal gene transfer can help to endow organisms with new functions," said Julie Dunning Hotopp, of the University of Maryland School of Medicine’s Institute for Genome Sciences.  "Once these genes take root in a new species, evolution can tinker with them to produce totally new abilities or enhance existing ones.  It is the biological equivalent of upcycling that happens in my Buy Nothing Group."

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An idea takes flight

There's a fundamental misunderstanding that even some people who understand and accept evolution have, and it's called teleology.

Teleology is the interpretation of events in terms of their final purpose.  Now, there are some things you can look at teleologically; at least some historical events, for example, occurred because someone (or a bunch of someones) had a goal in mind and purposefully drove toward it.

The problem is, evolution isn't goal-driven.  As I mentioned a couple of days ago in the post on the re-evolution of flightlessness in the Alhambra rail, it's the law of whatever works at the time.  But teleological thinking sneaks in all too easily -- all you have to do is to ask your average scientifically-minded fourth grader why giraffes have long necks, and you'll probably get the answer, "So they can reach high branches in trees for food."  This conjures up the image of a herd of short-necked giraffes looking longingly up at the tender, juicy foliage out of reach, and thinking, "Wow, sure would be nice," and over the next generations, that desire to reach the end goal of more food resulted in longer-necked giraffes being born.

It's a subtler distinction than it might seem at first.  The truth is that the variation comes first; the blind forces of mutation and recombination result in baby giraffes with varying neck lengths.  But the better food being higher up means that those with longer necks survive better, passing those genes on -- so over time, the population's average neck length increases.  No goal-driven, forward-thinking forces necessary; it's all driven by what is an advantage in the environment as it currently is.  Change the environment, and you change those selective pressures, and the population (if it has sufficient variability to do so) responds.

The situation is simple enough with giraffe necks (which is why every middle-school textbook on biology uses that as an example), but what about more complex structures?  This is when the subject of things like eyes and wings inevitably comes up, often along with the intelligent-design aficionados' favorite buzzwords -- "irreducible complexity."  A half an eye or half a wing, they say, isn't good for anything, so there has to be forward thinking, teleological design involved.  Modification of an arm into a wing only makes sense if there was intent, because the intermediate forms along the way are worse than what you started with.

There are two flaws in this argument.

The first is brilliantly described in Richard Dawkins's amazing book The Blind Watchmaker, which explains in clear layman's terms why the intelligent design argument doesn't work.  He takes the vertebrate eye as his example, which is admittedly an amazing device with dozens of working parts and thousands (if not tens of thousands) of kinds of proteins, all of which have to work together in order to generate the capacity for clear vision.  As Dawkins points out, though, all of these didn't have to evolve simultaneously -- that would indeed be hard to explain.  Starting with simple light-detecting eyespots (like in a flatworm), the structure evolved to become more and more complex, more and more sensitive, and all that had to happen is at each tiny step the improvement gave the animal an advantage over the previous form.  So the ID-proponents' claim that "half an eye isn't worth anything" is actually incorrect.  An eyespot (which isn't even half an eye -- maybe five percent of one) is clearly better than no ability to detect light at all.  And after that, each refinement made it better and was selected for, until finally you have a complex structure like your own eye, capable of color vision, light intensity accommodation, focusing, and depth perception.

The second flaw, though, is a fascinating one, and is the reason this whole topic comes up in today's post.  For something to be selected for, all it has to accomplish is to confer some sort of benefit on the organism -- not necessarily the one for which it will eventually be used.  This phenomenon, called preaptation (or preadaptation), is usually explained using the example of feathers and wings in birds.  The theory is -- supported by the presence of feathers in fossils of a number of species of dinosaurs -- that feathers evolved in the context of keeping warm (and possibly protecting the skin from sun exposure), and only afterward became useful for the ability of lightweight/arboreal species to glide, and finally to fly.

A second example of this was the subject of a paper last week in Nature, which I was alerted to by my friend, the amazing scientist and environmental activist Sandra Steingraber.  The topic was the hypothesis that preaptation had also occurred in insect wings -- they had evolved as gill extensions in aquatic larvae, and through minor modifications widened and lengthened, and in the adult became full-fledged wings.  The embryonic structures for larval gills and adult wings were certainly homologous, so it seemed like a good guess, but hard evidence was lacking...

... until now.

[Image licensed under the Creative Commons Michael Palmer, Yellow mayfly on leaf, CC BY-SA 4.0]

In "Genomic Adaptations to Aquatic and Aerial Life in Mayflies and the Origin of Insect Wings," a team led by Isabel Almudi of the Andalusian Center for Developmental Biology (in Sevilla, Spain) completely knocks it out of the park by identifying the genetic basis of both wings and gills in insects -- and demonstrates that the preaptation hypothesis was spot-on.  Here it is in the authors' words:

The evolution of winged insects revolutionized terrestrial ecosystems and led to the largest animal radiation on Earth.  However, we still have an incomplete picture of the genomic changes that underlay this diversification.  Mayflies, as one of the sister groups of all other winged insects, are key to understanding this radiation.  Here, we describe the genome of the mayfly Cloeon dipterum and its gene expression throughout its aquatic and aerial life cycle and specific organs.  We discover an expansion of odorant-binding-protein genes, some expressed specifically in breathing gills of aquatic nymphs, suggesting a novel sensory role for this organ.  In contrast, flying adults use an enlarged opsin set in a sexually dimorphic manner, with some expressed only in males.  Finally, we identify a set of wing-associated genes deeply conserved in the pterygote insects and find transcriptomic similarities between gills and wings, suggesting a common genetic program.  Globally, this comprehensive genomic and transcriptomic study uncovers the genetic basis of key evolutionary adaptations in mayflies and winged insects.

As Sheldon Cooper might say, "Bazinga."

This completely dismantles the "irreducible complexity" argument for insect wings, if there was any basis for that argument left.  The evolutionary model is vindicated again.  As Almudi et al.'s research shows, the genomic basis in an organism can be modified in such a way as to give structures multiple purposes -- and that if the environment is right, one purpose (e.g. flight) can supersede another (e.g. maximizing oxygen absorption) in importance, causing the evolutionary path to take off in a different direction.

So cheers to Almudi and her team.  I read their paper while grinning like a loon just because it was such a perfect wrap-up to a conjecture I was telling my AP Biology classes about twenty years ago.  It's so gratifying to study a model that has this kind of robust predictive power -- and further reinforces my opinion that in order not to accept evolution, you have to be willfully ignorant of the actual evidence in its favor.

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This week's Skeptophilia book recommendation of the week is a fun one: acclaimed science writer Jennifer Ackerman's The Bird Way: A New Look at how Birds Talk, Work, Play, Parent, and Think.

It's been known for some years that a lot of birds are a great deal more intelligent than we'd thought.  Crows and other corvids are capable of reasoning and problem-solving, and actually play, seemingly for no reason other than "it's fun."  Parrots are capable of learning language and simple categorization.  A group of birds called babblers understand reciprocity -- and females are attracted to males who share their food the most ostentatiously.

So "bird brain" should actually be a compliment.

Here, Ackerman looks at the hugely diverse world of birds and gives us fascinating information about all facets of their behavior -- not only the "positive" ones (to put an human-based judgment on it) but "negative" ones like deception, manipulating, and cheating.  The result is one of the best science books I've read in recent years, written in Ackerman's signature sparkling prose.  Birder or not, this is a must-read for anyone with more than a passing interest in biology or animal behavior.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]