The interesting things at Sandwalk always seem to happen when I’m not looking. On Sunday, while I was out west taking the offspring to start university at UBC, Larry Moran posted a blog on Constructive Neutral Evolution that has elicited almost 200 comments. Alas, many of the comments are not particularly useful, as Sandwalk is home to an ongoing pseudoscientific debate on intelligent design.
The one point that I would like to make about CNE is that it was not proposed as some kind of law or tendency (i.e., not like “Biology’s First Law” of McShea and Brandon). Some other people treat CNE as the manifestation or the realization of some kind of intrinsic tendency to complexification. If this were the case, then examples of reductive evolution (e.g., cases involving viruses and intracellular parasites) would raise a question about the generality of the idea. Obviously evolutionary change occurs in both reductive and constructive modes. Bateson and Haldane each speculated that reductive evolution would be common because it is so easy to accomplish.
From my perspective, CNE is not a theory about a general tendency of evolution. Instead it is a schema for generating specific testable hypotheses of local complexification.
One also can imagine a mode of Reductive Neutral Evolution in which simplification occurs. It is simply a matter of the local position of the system relative to the spectrum of mutational possibilities.