How Do Biologists Support Functional Claims?
The press has reported extensively on a new (and frickin’ cool) paper out in Science documenting gears on the back legs of a little insect (Issus). Ed Yong over at National Geographic has a piece on this work, including embedding the little movie of the gears in action, which is, as I say, 100% totally and completely cool.
Because the press has covered this work, I’m not going to talk about the work per se.
Rather, I want to address an issue which I’ve discussed from time to time, which is how biologists draw inferences about function. What kinds of evidence do they use when they make a claim that a particular structure is for something?
I’m not interested in this for purely philosophical reasons regarding the epistemic commitments of biologists as a general matter.
I’m interested in this because of critiques regarding functional claims made in the context of evolutionary psychology. In particular, critics have made various suggestions about what sort of evidence is used by biologists to make claims regarding function, suggesting that similar sorts of evidence ought to be used when evolutionary psychologists make claims about function.
So, how do Burrows and Sutton, the authors of the paper in question – “Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect” – provide evidence for their claim?
First of all, note the functional claim in the title. The crucial word there is “synchronize.” The function of the bits of the body in question – the little gears – are to synchronize the legs. If the legs weren’t in synchrony, then the insect’s jump would be asymmetric, and the insect would tilt to the left or right instead of jumping straight ahead. (I watched some videos of frogs jumping, and it looks to me that frogs use asymmetrical leg motions to (intentionally?) get some yaw into their jumps. Anyone know anything about that?)
Second, note the evidence that they present. Figure 1(D) is a picture, showing the interlocking gears. Figure 1(E) is a diagram of the gears. The Supplemental Material includes a series of movies. These materials also indicate the methods used: mostly making the movies and taking the photographs. The authors are not shy about their conclusions, writing:
The mechanical gears in Issus enhance the synchrony between leg movements to the level of microseconds…The gears in Issus… demonstrate that mechanisms previously thought only to be used in manmade machines have evolved in nature. Nymphal planthoppers have interacting gears that play an essential functional role in a natural behavior.
To be clear, then, the authors infer the function – synchronization of the legs, which in turn allows for jumping without any yaw – from the images. The shape of the structures they found, together with the behavior they observe, affords the inference regarding function. This is because those particular shapes – interlocking gears – are complex elements of the phenotype that are improbably well designed to execute the function in question, synchronization. As George Williams articulated so well in Adaptation and Natural Selection, functional complexity is the hallmark of adaptation. It’s worth quoting Williams, who specifically discussed cases like the one in the present paper, in which there is an analog with human artifacts:
A frequently helpful but not infallible rule is to recognize adaptation in organic systems that show a clear analogy with human implements. There are convincing analogies between bird wings and airship wings, between bridge suspensions and skeletal suspensions, between the vascularization of a leaf and the water supply of a city. In all such examples, conscious human goals have an analogy in the biological goal of survival, and similar problems are often resolved by similar mechanisms. Such analogies may forcefully occur to a physiologist at the beginning of an investigation of a structure or process and provide a continuing source of fruitful hypotheses. At other times the purpose of a mechanism may not be apparent initially, and the search for the goal becomes a motivation for further study. Adaptation is assumed in such cases, not on the basis of a demonstrable appropriateness of the means to the end but on the indirect evidence of complexity and constancy.
In short, both Williams and the present authors infer function from form. But note that there are important dogs that don’t bark here. The authors don’t provide any data regarding the genetics of the structures in question. They don’t provide any data regarding heritability. There aren’t any experiments. And that’s just fine. The point is that the shapes are themselves the evidence. The rows of teeth, interlocking as they do, provide compelling evidence for the function of synchronization.
Is there room for doubt? Of course. I find myself persuaded, but I suppose it’s possible that these teeth arose as a side-effect of some other aspect of the phenotype. That possibility seems wildly unlikely to me, but I suppose it could be. Or maybe the gears are actually some sort of sense organ. Again, that seems wildly improbably to me, but I suppose there’s some room for doubt. (Similarly, the emotion of jealously might be a side-effect of some other computational system, rather than a mechanism to deter infidelity. Could be.)
Is the claim that these structures are for synchronizing leg movements a “Just So” story? Does a structure’s shape, in itself, allow you to increase your confidence in a specific functional hypothesis? If you’re working in the realm of behavior, what is the equivalent of measuring and documenting the shapes of structures?
How do (these) biologists support their functional claims?