Several days ago I called attention to Richard Lewontin’s review of WEIT and several other books in The New York Review of Books. In it, Dick (excuse the informality, but he was my Ph.D advisor) praises the book but takes me to task for implying that the evidence for natural selection is as strong as the evidence for evolutionary change per se:
Where he is less successful, as all other commentators have been, is in his insistence that the evidence for natural selection as the driving force of evolution is of the same inferential strength as the evidence that evolution has occurred. So, for example, he gives the game away by writing that when we examine a sequence of changes in the fossil record, we can
“determine whether the sequences of changes at least conform to a step-by-step adaptive process. And in every case, we can find at least a feasible Darwinian explanation.”
But to say that some example is not falsification of a theory because we can always “find” (invent) a feasible explanation says more about the flexibility of the theory and the ingenuity of its supporters than it says about physical nature. Indeed in his later discussion of theories of behavioral evolution he becomes appropriately skeptical when he writes that
“imaginative reconstructions of how things might have evolved are not science; they are stories.”
While this is a perfectly good argument against those who claim that there are things that are so complex that evolutionary biology cannot explain them, it allows evolutionary “theory” to fall back into the category of being reasonable but not an incontrovertible material fact.
There is, of course, nothing that Coyne can do about the situation. There are different modes of “knowing,” and we “know” that evolution has, in fact, occurred in a stronger sense than we “know” that some sequence of evolutionary change has been the result of natural selection. Despite these misgivings, it is the case that Coyne’s book is the best general explication of evolution that I know of and deserves its success as a best seller.
This “critique” has been picked up by several bloggers (see below), and I want to respond in a bit more detail.
First of all, yes, it’s true that the evidence for natural selection as the cause of most evolutionary change in the past is not as strong as the evidence that evolutionary change occurred. It cannot be otherwise. We can see evolution happening in the fossil record, but it is infinitely harder to parse out the causes of that change. We weren’t around when it occurred, so we must rely on inference. This difficulty is one reason why it took biologists much longer to accept natural selection than to accept evolution. But to say that the evidence for selection is weaker than for evolution does not mean that the evidence for natural selection is weak, a conclusion I fear that creationists will extract from Lewontin’s comment.
Here is why selection still seems the best hypothesis for the origin of adaptive features of organisms.
1. It is the only scientific theory, among all of those that have been adumbrated, that currently makes sense. Failed explanations include teleology, intelligent design, and Lamarckism. Some of these were once valid scientific alternatives to natural selection, but have failed either because they are untestable or because they were testable and shown to be wrong. If Lewontin and others want to say that some process other than selection is responsible for the limbs of tetrapods, the fins of whales, and the white color of polar bears, they must say what they envision. Yes, Lewontin and Gould showed that many things for which we can concoct adaptive stories may be “spandrels” — nonadaptive traits hitchhiking on other adaptations — but this does not mean, as Lewontin seems to imply, that selection may not play a major role in creating adaptations.
2. In cases where we can actually investigate whether selection is responsible for an adaptive change in a species, it is. I give several examples in WEIT, including coat colors in mice and the famous work of Rosemary and Peter Grant on Darwin’s finches. And of course there are those dozens of cases of antibiotic resistance in bacteria, insecticide resistance in arthropods, and herbicide resistance in weeds. In bacteria, for instance, we can show that the genetic variation for resistance preexisted in the population and not invoked by the selective agent, precisely as the theory of natural selection posits.
3. In tests where we envision that selection was responsible for an adaptation, we can do laboratory tests to see if the adaptation at least gives a fitness advantage to those individuals possessing it. One example of this is the Browers’ work on Batesian mimicry in the viceroy butterfly. It was shown that exposure to a toxic monarch made naive bluejays sick, and that later these bluejays avoided the nontoxic viceroys, giving a survial advantage to mimics. This is precisely what has to happen for that mimicry to evolve by natural selection. Likewise with color in guppies: brightly colored guppies get eaten more often in Trinidadian streams than do their duller confreres. This explains why guppies are less colorful in predator-filled streams.
4. The prerequisites for selection — the heritability of traits, the fact that there is competition between individuals, and that there are fitness differences between individuals with different traits — have all been demonstrated in living organisms in nature. If few traits showed any heritable genetic variation, we’d be justified in rejecting selection as a major cause of evolution. Guppy coloration is heritable.
5. Even in ancient species we can test the likelihood that selection caused evolutionary change. Horses lost their toes right about the time when the forests were disappearing on the Great Plains. We know that hooves are more effective adaptations for running in open grassland than are multi-toed feet. Likewise, horse teeth become higher and more robust precisely when silicon-rich grasses were replacing the leafy forests. We know that herbivores need higher and more robust teeth to deal with grass. It is a good inference that the appearance of grassland was the selective factor promoting the loss of horse toes and the change in horse teeth.
6. As discussed in previous posts, selection as we envision it has certainly been adequate to explain the evolution of complex adaptations like the eye, and in geologically reasonable periods of time. Therefore it remains a viable hypothesis for adaptive change. This didn’t have to be the way it turned out.
What about my supposed double standard about accepting natural selection for many traits but being skeptical when it comes to evolutionary psychology? This is a reasonable tactic for one important reason: we have many more alternative theories for the appearance of human behavioral traits than we do for morphological adaptations in other species. How many alternative theories do we have for the appearance of flippers in proto-whales, or for the movement of their nasal passages to the top of their heads? In contrast, there are many alternative theories for the appearance of traits like human rape, depression, music, art, religion, etc. Blowholes aren’t likely to be spandrels; the appearance of music and poetry might well be. Humans have culture and rationality to a degree possessed by no other animal, and can learn many things not permitted in species having smaller (or no) brains. That’s why we need to be more cautious about imputing selection to human behaviors than to blowholes.
Now I think Dick did have a point: I should have pointed out (though I might have; I can’t remember!) that it is a lot easier to come up with evidence for evolution than for selection. But I think Lewontin’s own anti-selectionist biases are intruding here. As I mentioned in an earlier post on ideological grounds neither he nor Gould were ever very strong promoters of selection. I’m not sure what the connection is between selection and politics (it may be the misuse of selectionism that Gould and Lewontin saw among sociobiologists), but neither of these chaps were avid promoters of selection. They preferred to emphasize other processes, including pleiotropy, spandrels, genetic drift and the like. I think this was a deliberate strategy.
Over on EvolutionBlog, Jason Rosenhouse analyzes Dick’s review and has some good comments:
I don’t understand what it means to say that “natural selection is the driving force of evolution.” Given Lewontin’s past writing (most notably his spandrels paper with Stephen Jay Gould) I would guess that his point is that some biologists are too quick to attribute some anatomical feature of some organism to the prolonged working of natural selection.
That may be true, but when we are talking about adaptations the evidence for natural selection seems to me to be very strong. For one thing, it is the only natural mechanism known that can account for complex structures (like bird wings or vertebrate blood clotting systems). For another, every complex structure studied to date shows clear evidence of being a cobbled together Rube Goldberg machine, which is exactly what we would expect if they were crafted by natural selection.
On top of this, biologists routinely use adaptive reasoning to generate testable hypotheses about the creatures they are studying. Lewontin would know better than I whether biologists engaging in flights of fancy is a genuine problem in the field, but it is undeniable that “the adaptationist program” has yielded great dividends over the years . . .
. . In fairness, I think Stephen Jay Gould was pretty clear on this point [the ubiquity and importance of selection] in several of his essays. I compiled some of his statements on the matter in this essay. But I share Coyne’s frustration. I’ve never really understood what it is exactly that anti-selectionists are complaining about. If they agree that complex adapations arise as the result of gradual accretion mediated by natural selection, then I fail to see how they are really so different from people like Richard Dawkins or Daniel Dennett (two people often described as being beknighted uber-selectionists). If they do not agree then I would like to hear their proposed alternative mechanism.
Now I have great affection for Lewontin (as all of his students say, “I love that man”), but I would like to see him make an explicit statement about what aspects of nature he imputes to natural selection. We’re not just talking about rape, male domination, and music here, but coat colors, physiology, feathers, gills, flowers, toxins, and the like. Like Jason, I think the anti-selectionists have gone way, way overboard, and have thrown out the baby with the bathwater. (These people also include the “structuralists,” and those who attribute adaptations to the self-organizing properties of biological matter.)