Some time ago, I gave a seminar about some work involving chicken combs, and I made some offhand remark about how I don’t think that the larger combs of modern layer chickens are the result of direct selection. I think it is more likely to be be a correlated response to selection on reproductive traits. During question time, someone disagreed, proposing that ornamental traits should be very likely to have been under artificial selection.
I choose this example partly because the stakes are so low. I may very well be wrong, but it doesn’t matter for the work at hand. Clearly, I should be more careful to acknowledge all plausible possibilities, and not speculate so much for no reason. But I think this kind of thing is an example of something quite common.
That is: researchers, even those who fit snugly into the same rather narrow sub-field, may make quite different default assumptions about the world. I suspect, for instance, that we were both, in the absence of hard evidence, trying to be conservative in falling back on the most parsimonious explanation. I know that I think of a trait being under direct selection as a strong claim, and ”it may just be hitch-hiking on something else” as a conservative attitude. But on the other hand, one could think of direct artificial selection as a simpler explanation as opposed to a scenario that demands pleiotropy.
I can see a point to both attitudes, and in different contexts, I’d probably think of either direct selection and pleiotropy as the more far-fetched. For example, I am hard pressed to believe that reductions in fearfulness and changes in pigmentation of domestic animals are generally explained by pleiotropic variants.
This is why I think that arguments about Occam’s razor, burdens of proof, and what the appropriate ”null” hypothesis for a certain field is supposed to be, while they may be appealing (especially so when they support your position), are fundamentally unhelpful. And this is why I think incommensurability is not that outlandish a notion. Sometimes, researchers might as well be living in different worlds.