Constructive neutral evolution on Sandwalk
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.
TDPB
January 7, 2019 - 4:26 pm
Good points all around. The notion of “Laws” in biology is already so dubious that it would be a shame to hitch CNE to that (possibly dead) horse.
Nonetheless, I don’t believe that reductive evolution necessarily counts against the generality of CNE. Often, we can look at examples of RNE as CNE at a higher level, such as when a community of organisms supports (has an excess capacity to support) individuals with reduced genomes. This, I think, is CNE at the level of communities supporting RNE at the level of individuals.
Indeed, this does not make CNE any more law-like, but it does equip it with some features of a general theory. Happily, it is precisely that aspect of CNE that allows it to serve as a means of generating (sometimes testable) hypotheses about complexification (when relativized to a given level of organization).
(*shameless self promotion to follow*) W. F. Doolittle and myself recently argued as much here: https://link.springer.com/article/10.1007/s10539-018-9614-6
Arlin Stoltzfus
April 18, 2019 - 9:53 pm
Thanks for your comment. Yes, reductive evolution does not count against CNE, but counts against the notion of a principle of intrinsic complexity increase. I hope to read the cited article soon.
Jon Perry
July 13, 2022 - 4:30 am
The work of J.W. Thornton seems extra useful here. He ressurects consensus sequences proteins to study the origins of various protein complexes. Super cool idea!
One of his papers to show a clean case of CNE (but he seems unaware of the term). What he found is even more extreme than fully neutral CNE. He showed that degrading mutations (after duplication) can be the cause of an obligate increase in complexity. Here’s the paper:
Evolution of increased complexity in a molecular machine. Nature, 09 January 2012.
I recently read another paper by Thornton & friends exploring the origin of hemoglobin.
In this case, I don’t know that we have a clear example of CNE, (mutations all seemed to cause immediate functional changes that, depending on the fitness landscape, could have been selected for). Anyway, give it a look and see if I missed something:
Origin of complexity in haemoglobin evolution. Nature, 20 May 2020
Last but not least, the Lenski lab seems to talk about CNE (or something very similar) but by a different name: Potentiating Events. That said, it seems they leave room for potentiating events to be at least mildly beneficial in & of themselves, so long a they don’t trigger the “actualization” of a key innovation.
Hope this added something useful to the conversation.