Thesis of the post: "...for functional traits, null hypotheses for variation among species should be adaptive ones (such that the non-adaptive hypothesis bears the onus of proof), whereas null hypotheses for variation within populations should be non-adaptive ones (such that the adaptive hypothesis bears the onus of proof)"
I can see how this would make sense. I suppose it is more likely that differences between two separate populations are caused when each adapts to its separate environment. It seems this would only apply for allopatric populations. On the other hand, if there are differences within a population, you could argue it is more likely that this is just due to random variation or phenotypic plasticity. So the null hypothesis for differences between populations is that they adapted to different environments, and the null for differences within populations is that they have plastic traits.
I feel this conclusion largely relies on the assumption that allopatric speciation is simpler and much more likely to occur. Which may not be the case. As one commenter put it: "Simplicity always has to depend on the context and is therefore not general."
Why couldn't some Mytilus californianus in a group adapt
and change more than others?
Why couldn't there be speciation within a tight cluster
of Pollicipes polymerus?
This idea is not fully developed in my head yet, but it is something to think about! Thanks, Joseph.
covered in Balanus glandula