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The Improbability Pump | The Nation

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The Improbability Pump

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Could biologists really have been so mistaken these past 150 years? It seems unlikely, but scientific consensus has been wrong before (the idea of continental drift, for instance, was once widely rejected), so let's examine F&P's claims. These fall into two groups. The first is that scientists have recently discovered a lot of things about genetics and development that make natural selection look ineffective:

The Greatest Show on Earth
by Richard Dawkins
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What Darwin Got Wrong
by Jerry Fodor and Massimo Piattelli-Palmarini
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About the Author

Jerry A. Coyne
Jerry A. Coyne is a professor of ecology and evolution at the University of Chicago. His latest book is Why Evolution...

Contrary to traditional opinion, it needs to be emphasized that natural selection among traits generated at random cannot by itself be the basic principle of evolution. Rather, there must be strong, often decisive, endogenous constraints and hosts of regulations on the phenotypic options that exogenous selection operates on.

In other words, Darwin's assertion that species are "quite plastic" is wrong: organisms are so constrained by their biological nature that they're not free to change, even if it would be good for them to do so. So what are these "constraints" and "regulations" that render natural selection impotent? F&P proffer a long list (I count at least two dozen items), including the following phenomena: horizontal gene transfer (movement of DNA between individuals of the same generation and between different species), alternative splicing of genes, robustness, modularity, molecular drive, entrenchment, developmental noise, phenotypic plasticity and self-organization. To the layman, this salvo of arcane terms is daunting, and even I, an evolutionary geneticist of forty years' standing, was taken aback. But not for long, because on close inspection we find that none of these phenomena put much of a brake on natural selection.

Let's look at one of these: phenotypic plasticity. This refers to the ability of a phenotype--an observable trait or characteristic of an organism--to change within a single generation in response to environmental fluctuations. This is what happens, for instance, when you get a tan. If you have an outdoor cat, its fur gets thicker in winter. The plumage of Arctic animals like the ptarmigan, ermine and Arctic hare changes color from brown to white as winter comes on. Even the lowly brussels sprout has sophisticated plasticity: when it detects that a sprout-eating butterfly has laid eggs on the plant, it changes its leaf chemistry to attract parasitic wasps that destroy those eggs.

F&P imply that somehow--they're not clear about how--this ability to undergo adaptive developmental change within a generation prevents natural selection from causing genetic change between generations. But that isn't the case. In fact, far from being an impediment to natural selection, the ability of an individual to adapt to a changing environment is a product of natural selection! Individuals who can tan in the sun (and thus prevent melanomas) have an advantage over those whose pigmentation is fixed. Cats are better off if the length of their fur suits them to the seasons. Genes that are able to respond to predictable variation in the environment will always outcompete those that produce only a fixed (and hence episodically maladaptive) trait.

And so it is with all the "constraints" on selection detailed by F&P. A reader lacking training in science might skim over the rather tedious discussion of these phenomena and assume that F&P know what they're talking about. That reader would be wrong. Look at it this way: if there really were so many constraints on selection, and if development really were so complex and tightly interconnected that organisms could not respond to natural selection, then why would artificial selection be so effective at changing animals and plants?

Indeed, virtually none of the biologists who study the "constraints" described by F&P share their dim view of natural selection. That's because, over and over again, selection has wrought the most improbable and unpredictable changes in animals and plants. F&P claim, for example, that selection could never produce winged pigs because of developmental constraints: "Pigs don't have wings because there is no place on pigs to put them. There are all sorts of ways you'd have to change a pig if you wanted to add wings. You'd have to do something to its weight, and its shape, and its musculature, and its nervous system, and its bones; to say nothing of retrofitting feathers."

Haven't F&P heard of bats? Bats evolved from small four-legged mammals, probably resembling shrews. You could say the same thing about shrewlike beasts that F&P did about pigs: how could they possibly evolve wings? And yet they did: selection simply retooled the forelegs into wings, along with modifying the animal's weight, shape, musculature, nervous system and bones for flying (no feathers needed). One of the great joys of being a biologist is learning about the many species in nature whose evolution would appear, a priori, impossible.

Beyond distorting the scientific literature, F&P make a number of claims that are simply silly. I mention just one: "The textbook cases of Mendelian inheritance, in spite of their great historical and didactic importance, are more the exception than the rule." This came as a surprise to me. In fact, cases of Mendelian inheritance (the random assortment of parental genes into sperm and eggs) are the rule; if they weren't, genetic counseling would be useless. Statements like this typify the authors' attitude toward science throughout their book: they seize on some new wrinkle in the scientific literature, like a rare gene that doesn't behave according to Mendel's rules, and interpret it as a revolution that nullifies all of mainstream biology. This lack of grounding is often seen in work by science journalists who make their living touting "revolutionary" new findings, but it is inexcusable in a supposedly serious book written by academics.

Given F&P's expertise, you'd expect them to be on firmer ground with their second objection to natural selection, which is philosophical. But again they founder, making illogical arguments and distorting how biologists work. Natural selection is philosophically incoherent, they claim, because it doesn't "support counterfactuals." (A counterfactual is a conditional statement about what hasn't happened but could if certain conditions were met. The paradigmatic example is Tevye's song "If I Were a Rich Man," from Fiddler on the Roof.) What F&P mean is that in real organisms, evolution often involves simultaneous changes in several features, and we simply don't know which changes reflected natural selection (that is, which traits had variation that directly affected survival or reproduction) and which traits were what they call "free riders": features that weren't subject to selection but were carried along, perhaps as nonadaptive byproducts of genes that evolved.

Here's an example: during the evolution of mammals from reptiles, several features changed at the same time. The limbs moved underneath the body, the teeth became differentiated, some jawbones shrank and the braincase got bigger. Now, which of these traits evolved by natural selection, and which, if any, might have only been byproducts? Maybe selection acted to enlarge the cranium but the reduced jawbones were only a passive, nonselected byproduct of "braincase" genes.

Even single traits have several effects that could, in principle, respond differently to natural selection. The hemoglobin in our blood, for example, carries oxygen but also happens to be red. How do we know that selection didn't favor the color itself (perhaps to make anger or blushing more evident) rather than the oxygen-transport ability?

Finally, some "traits" are inseparable even in principle: how can we tell whether the ancestors of whales experienced selection for "swimming" as opposed to "flapping their flukes up and down?" Fluke-flapping, of course, causes swimming. Evolutionary biologists aren't much concerned with this sort of distinction, and I won't consider it further.

Because biologists can't make any of these distinctions, say F&P, natural selection is incoherent. They call this the "selection-for" problem. And they add that artificial selection doesn't suffer from this problem. Since animal and plant breeders consciously select for certain traits, like higher milk yield or uglier bulldogs, we know exactly which features experience selection (the bulldog's puggish face) and which are byproducts (the respiratory problems that come with puggish faces). F&P therefore find Darwin's analogy between artificial and natural selection dubious, for "only minds are sensitive to distinctions among counterfactuals," and "natural selection doesn't have a mind." In the end, declare F&P, natural selection cannot be true because "a theory that doesn't determine the truth values of relevant counterfactuals cannot explain the distribution of traits in the actual world."

But wait a minute. If you translate that last sentence into layman's English, here's what it says: "Since it's impossible to figure out exactly which changes in organisms occur via direct selection and which are byproducts, natural selection can't operate." Clearly, F&P are confusing our ability to understand how a process operates with whether it operates. It's like saying that because we don't understand how gravity works, things don't fall.

 

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