28 May 2011

“I’m a Scientist, Nothing Shocks Me.”

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That may not be an exact quote, but that’s a line from Indiana Jones and the Temple of Doom. I was reminded of it when reading David Friedman’s recent blog post on evolutionary theory and new evidence that a low-salt diet might be bad for you. Friedman wrote:

There is a longstanding argument for reducing the amount of salt modern Americans consume, based on evidence that a high salt diet tends to produce high blood pressure. A recent European statistical study, however, reported just the opposite of what that argument suggests—evidence that lower salt intake was correlated with an increased risk of death from heart disease. Similarly, there is evidence that an increased consumption of omega 3 oils reduces the risk of heart attacks. But it has recently been reported that it also increases the risk of the more serious form of prostate cancer.

The logic of optimization provides an explanation for these results. The human body, like the race car, is a machine optimized for a purpose, although the optimization is by evolution rather than deliberate design. If it functioned better with less salt, the design would at some point of have been tweaked to consume less salt, excrete more salt more rapidly, or in some other way take advantage of that particular opportunity for improved design. If it functioned better with whatever metabolites fish oil produces, the very sophisticated chemical factory build into our metabolism would, presumably, have been modified over time to produce those metabolites without requiring that particular input. It is not surprising if changes produce improvements on some dimension of successful functioning for the human organism—but it is also not surprising if those changes, like changes in the design of a race car, produce at least equal worsening on other dimensions.

I do not want to overstate the point; there are at least two reasons why the design of my body might be suboptimal from my point of view… [Friedman then explains the two major caveats to his argument.–RPM]

But the implication of the argument I have offered is that we ought not to be surprised by results such as the two I just discussed. The fact that some change produces a gain in one measurable dimension that matters to us is very poor evidence that it produces an overall gain.

OK kids, I want you to control yourselves. Do NOT start talking to me about the Flying Spaghetti Monster, because I’m not challenging evolutionary theory per se in this post. All I am saying is that Friedman’s conclusion, well, surprised me. Yes, he’s right as far as he goes, but I could use evolutionary theory to argue that no matter what, we shouldn’t be surprised.

Friedman has just shown how evolution can explain why tweaking things can lead to problems even worse than the original thing.

But hey, I can use glasses to improve my eyesight, and as far as I can tell being able to see better doesn’t cause me to go deaf and get bitten by a rattlesnake. So that’s a blow against evolutionary theory, right? I mean, if it made sense for me to have better eyesight and this wouldn’t compromise something else, then why didn’t my ancestors weed out my shortsightedness?

Ah no, evolutionary theory would lead us to expect such a result! You see, there are constraints on the mechanism (which Friedman spelled out nicely in the part I omitted above for brevity). Actually, seeing the imperfections in living organisms is yet another feather in Darwin’s cap. Why would a benevolent God give His children birth defects, after all?

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My point is simple: If we accept Friedman’s argument–and I honestly have no problem with it, as far as it goes–then we must realize that no matter what, we “shouldn’t be surprised” by such a result. If all the best medical evidence suggests that taking a multivitamin every day, or getting double bypass surgery at age 10 for that matter, confers far more benefits than damages, that by no means is a blow to evolutionary theory. In fact, most believers in evolution would find their theory vindicated yet again.

Thus my point: No matter what–in the context of the sorts of things Friedman is discussing–we “shouldn’t be surprised” at how modern organisms behave, in light of evolutionary theory. Note that I’m not claiming the full-scale evolutionary theory is non-falsifiable. I’m saying that it is, for the use to which Friedman is putting it in his blog post.

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