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What Good Is Half A Brain

I recently wrote a post for NeuroLogica in which I discuss the latest transitional fossil find – a pinniped (seal, sealion, walrus) ancestor that was able to walk on land (a walking seal). One commenter simply asked “what good is half a flipper.” At first I assumed the question was tongue-in-cheek, but on second thought I guess I can’t know if it is serious or not. Either way, this is a common creationist point, and like most of their points it is one they continue to make despite the fact that it has been answered for decades. In fact, Darwin himself gave a reasonable answer to this anticipated objection in The Origin of Species.

The premise behind the question is that in order for a feature to be selected for in a Darwinian sense it would have to provide some advantage to an organism. Evolutionary forces cannot plan ahead or look to the future – they cannot select for a feature because it is on its way to becoming something useful – it has to be useful every step of the way.

This was a good point 150 years ago, but as they often say in courtroom dramas – it has been asked and answered. There are at least five phenomena that contribute to an explanation for the “half-formed” problem in evolution.

The one that applies to the half-flipper question, actually, is the easiest – sometimes a structure does provide an advantage every step of the way. A half-flipper is, in fact, a perfectly useful flipper. With something like flight there is a threshold a creature must get to in terms of morphological features before they can fly at all. Therefore a wing cannot be selected for only because it will eventually be used for flight. But this is not true of swimming.

With swimming there is a continuous spectrum from barely being able to keep from drowning to highly adapted, fast and acrobatic swimming. Each tiny incremental change in morphology equals a tiny incremental improvement in swimming, and therefore can be selected for. So the dilemma does not even exist for the case of a “half flipper.”

The second phenomenon, which Darwin himself pointed out, is called exaptation or cooption. What this means is that a structure (or behavior, or biochemical pathway, etc) that originally is adapted to one function is used oportunistically for another function. With the classic example of a wing – a half-wing may be no use for flying, but a half-wing may be a fully formed something else. Therefore theropod dinosaurs may have evolved downy feathers for insulation. These feathers then were used for display, bringing in a new set of selective forces. The display feathers then may have been coopted for slowing a descent from a high jump, or extending the length of a predatory jump, or even for trapping small animals or insects. The new function then puts new selective pressures on the feathers and protowings, which are then, for example, optimized for gliding. The gliding wings can then be coopted for full flapping flight.

There are therefore plausible scenarios for the evolution of a wing useful for flying, without having to pass through a useless half-wing stage.

We would expect, in fact, evolution to take such chaotic and quirky paths, making it difficult to “reverse engineer” any structure or pathway that is the end result of a long and winding evolutionary path.

Of note, Behe’s irreducible complexity (the notion that a structure is irreducibly complex if it could not function if it were any simpler) is just a restatement of this old evolutionary dilemma, solved as above by Darwin himself. Behe ignores exaptation. For example, in his infamous bacterial flagellum example – a simpler flagellum may not function (actually this too is not true, but for the sake of argument), but there is evidence that it evolved from simpler structures that were used to inject toxins into other organmisms.

There are three other evolutionary phenomena which could solve the problem of how to evolve a complex structure that could not function for its current utility until it had reached a certain minimal level of complexity. One is genetic drift. Populations are always generating new biodiversity from mutations and recombination. Any features not specifically selected against could arise, survive, and undergo further modification. This change in gene frequencies or propagation of new varities is called genetic drift, because it is not being forced in one direction by selective pressures.

Therefore, structures and pathways that arise by chance alone may, at times and quite by accident, be coopted for some new behavior or ability. Even if it provides a crude tool and a marginal advantage, it can be selected for.

There is also a phenomenon known as gene duplication. Single genes, or sometimes even sections of a chromosome containing many genes, or even entire chromosomes, may be duplicated in the reproductive process. If a creature has two forms of a necessary gene, one copy can retain the original function, while the second is free to mutate and undergo genetic drift. It is free to experiment and may hit upon some new useful function. Once coopted, this new function will then be under selective pressures and can be refined.

Finally, it must be recognized that not all changes are incremental or minimal. There are regulatory genes that can have profound effects on the course of development. This is the “hopeful monster” hypothesis – that most such profound changes are likely to be detrimental, but evolution has millions of years and billions of organisms to work with, so it only takes an extremely rare beneficial change to create sharp turns in the path of evolution.

Taken together these phenomena illustrate that evolution is not a clean, predictable, steady process – but rather a chaotic, unpredictable, historically contingent one. Weird, unlikely, and complex stuff happens because of the amount of time and number of individual organisms involved. In fact, the more we understand about evolution the more complex it seems.

While we continue to drill deeper in the science of evolution, the fact of evolution becomes more and more solid over time. Meanwhile, creationists continue to raise the same old objections to evolution that have already been answered. Sometimes they rephrase them, trying to get another round of debate out of a dead issue, but usually they just repeat their debunked nonsense. This one – what use is a half-whatever – will likely be around as long as there are creationists.

4 comments to What Good Is Half A Brain

  • The Blind Watchmaker

    What use is half a brain?

    (insert punch-line here)______________________

  • akronnick

    You probably could get elected to the United States House of Representatives, or maybe the Texas School Board.

  • gr8googlymoogly

    Half a brain would make me the smartest (by FAR!!!) creationist.

  • johntheplumber

    Stephen Novella sir, you say that “Not all changes are incremental or minimal.” Is this is one step from accepatance of a genetic leap – and there is no doubt that such a leap would help make the incremntal leap from nearly flying to full blown flight. The problem being of course how do you test whether you can fly properly or not without killing youself. It is easy to imagine scenarios of improvement, clearly things fly, I believe flight evolved, but this simple point can only be answered with complex imagined scenarios. I ask a different question but in the same vein. How do creatures that dodge become better at dodging? If you are a good dodger you live to reproduce – a poor dodger is soon dead trying. How do you evolve a good dodger. Again, a genetic leap from ‘nearly able to dodge’ to ‘a good dodger’ would come in handy.
    Forty years ago I discovered a natural function of environment that allows a genetic leap. – It’s there if you look for it.

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