Constructive Neutral Evolution 20 Years Later

The topic of how evolution can lead to complexity has been of interest…from all sides. Those see complexity in biology as the result of millions of years of evolutionary tinkering via natural selection, whereas there are those who see (some examples of) complexity to be an impassible barrier to the processes of natural selection. It’s the argument from irreducible complexity. Remove 1 (or a number of) parts from the system and it won’t work. Thus, is simply no way for such level of complexity to have evolved incrementally step-by-step with each step being increasingly beneficial and thus favored by natural selection. However, there are lot’s of issues with this arguments as many have pointed out. The most common example is provided by Kenneth Miller’s conceptual example of the unfashionable tie-clip. An incomplete mouse-trap cannot function as a mouse trap, but it can function for other purposes. Thus complexity can evolve via natural selection if you allow for functions to change. There is also the example of the scaffold, that is later removed. In this case, the arch is the conceptual example. It is a self-sustaining structure where the removal of one part will result in the collapse of the whole system. Yet, it is possible to build it one block at a time providing you include the scaffold in the first phase. The scaffold is simply omitted when ID proponents argue that a system cannot evolve step-by-step simply because it will cease functioning if you remove one (or a few) parts from the system.

But there is another evolutionary process which - I would argue - is overlooked by both sides. Constructive Neutral Evolution (CNE), proposed by Arlin Stolzfus in 1999.

It is is not commonly talked about, which is rather strange since the concept is rather straightforward. Suppose you have a protein that does a particular function. Then, another protein binds to it. But the first protein can function properly without it. Now, later, the first protein mutates such that it is no longer able to function without the extra protein. Now the system has become more complex and every part of the system is necessary to function, yet - functionally speaking - nothing changed.


The key step here is when the extra part becomes indispensable. This has an effect of a ‘ratchet’. Complexity is allowed to increase, but then it is prevented to be decreased. It’s a one way streat. This is CNE. Complexity increases but the result on function is ultimately neutral / non-adaptive. The historical role of CNE would explain the existence of rube goldberg systems in biology. Those that seem so unnecessarily complex than what would be actually required for the functions. It would also provide another way for irreducible complexity to evolve. Not by natural selection, but by neutral evolution as well. And the two are not mutually exclusive. It turns out that both were required fro the evolution of our hemoglobin. As we all know, oxygen is carried by hemoglobin in our red blood cells. The hemoglobin is composed of 4 subunits. Originally, the ancestral Globin protein was a single subunit protein, like myoglobin is (a related protein). Thus, the evolution of hemoglobin has been reconstructed to have gone through a 2-subunit stage before it became the 4-subunit we know today. It turns out that the 2-subunit stage was likely largely neutral. An example of CNE. Then later, when it became the 4-subunit structure, this allows for new beneficial properties to emerge.

For more, see the recent review paper on CNE.

Also, happy new year everyone!


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