Over at Jerry Coyne’s blog, biologist Greg Mayer wrote:
One of the most important lessons of comparative anatomy is that evolution usually proceeds by the modification of pre-existing structures (or, stated more precisely, the modification of the pre-existing developmental programs that produce those structures). Certain changes are easier to evolve because the developmental system can be modified to produce them—evolution follows the developmental path of least resistance. In terms of the skeleton of vertebrates, this means that most evolutionary changes are reduction, fusion, loss, lengthening, shortening, thickening, and narrowing of bones. Evolution uses what’s already there, and rarely do wholly new structures arise. (from “Tinkering with elephants’ feet”)
All of this is true, yet we can proceed beyond this conventional thinking and ask a couple of questions:
WHY does evolution follow the developmental path of least resistance?
WHAT are the implications of evolution following the developmental path of least resistance?
As for the first question, that evolution follows the path of least resistance can be seen clearly from the fact that “Evolution uses what’s already there, and rarely do wholly new structures arise” and “evolution usually proceeds by the modification of pre-existing structures (or, stated more precisely, the modification of the pre-existing developmental programs that produce those structures).” But why is this? Maybe it has something to do with “information.”
Typical ID-evolution debates often involve claims of some protein or some gene having too much “information” to have been generated by chance. Yet perhaps those debates are misguided, as they tend to force us into two false conclusions – no protein/gene can arise from random processes or all proteins/genes arose from random processes. Perhaps the truth is in the middle. That is, evolution follows the path of least resistance because you do not have to generate as much information by tweaking something that exists as opposed to generating something from scratch. So, life could have begun with a set of genes/proteins that were not spawned by random processes, but from then on, this set has been expanded and modified by evolution. Isolated examples of proteins/genes generated by random processes could be incorporated if they fit the context of this initial non-random state.
So what are the implications? First, in some sense, life is a “high information” state. The path of least resistance in this case would be to modify the high information content rather than replace it. The low information states that are spit out by random processes cannot compete with the high information state that already exist. If life was a low information state, there would be no tendency for evolution to “use what’s already there.” Random processes could regularly generate proteins/genes that would replace what’s already there. In other words, if life was a low information state, there would be no or little deep homology, which, as we have seen, is what conventional neo-Darwinian thinking falsely predicted.
Second, that evolution follows the path of least resistance means that evolution has a direction. It is not a free-for-all, where all that matters is that something works to somehow enhance fitness. What’s more, this path of least resistance is tied to what is already there. The pre-existing structures and features of life pave the path of least resistance. Since evolution is tied to and guided by these pre-existing structures and features, we once again see just how strikingly plausible it would be to design evolution via front-loading.
In summary, while mainstream thinking acknowledges and promotes the idea that evolution follows the developmental path of least resistance, if we pause to think about it, this is not some brute fact, but an echo of the teleological essence of evolution.