|Newly-hatched salmon from my days of studying egg (and offspring) size.|
So, why would I think this argument was nonsense?
1. Who decides which explanation is the simplest – and by what criteria? What usually happens is that investigators have a favorite explanation already in mind and find ways to argue that it is the simplest. But someone else could easily have a different interpretation regarding simplicity. For instance, genetic drift is often invoked as a more parsimonious explanation than selection – kind of like a null model that needs to be rejected before one can invoke the more “complicated” interpretation of selection. I would argue precisely the opposite. As far as I can tell, no study has conclusively confirmed genetic drift (as opposed to selection) as a driver of among-population variation in functional traits – the only sorts of traits for which we would try to infer adaptation anyway. By contrast, countless studies have found strong evidence for adaptive divergence in such traits. Moreover, adaptation by natural selection is a simple logical (mathematical) outcome when genetically based traits are related to organismal fitness. Natural selection thus seems to me a much more parsimonious explanation than drift, which additionally requires very small population sizes and very weak selection! To paraphrase Bret Weinstein as quoted in The Tapir’s Morning Bath (p. 300): Adaptation is a better explanation than God, but God is a better explanation than drift.
2. Who’s to say that nature is parsimonious anyway? Consider for a moment the frequent use of parsimony as a formal method for considering the evolution of traits on phylogenetic trees. In essence, the model of evolution that requires the fewest transitions between character states is assumed to be the correct one - and this is certainly the simplest (most parsimonious) model. To me this idea seems crazy as a general assumption. Many cases are known of all sorts of strange, variable, and contradictory evolutionary changes in a given group of organisms. That is, evolution does not follow a simple linear progression through a set of states but is instead rife with all sorts of fits and starts and reversal. So, in this sense, it seems certain that parsimony will give you the wrong answer. (Although it isn’t guaranteed that any other methods will give you the right answer).
All these thoughts were going through my head as I examined Njal and I couldn't help myself from going on a long diatribe along these lines. At the end, I asked Njal “So ... do you agree?” To which he responded “Well, now that you have brow-beaten me into it.” So, yes, I hope to have also now brow-beaten you into it too. Parsimony is a lousy approach on which to base scientific – and certainly evolutionary – inference, unless of course you believe that adaptation by natural selection is the most parsimonious explanation.