I started this post thinking I’d write a review of Andreas Wagner’s recent book “Arrival of the Fittest: Solving Evolution’s Greatest Puzzle” (links below), an engrossing book about how biological innovation arises from the structure of metabolic, genotype, and protein networks, and how robustness–the stability of phenotypes in the face of underlying genetic variability–is critical in evolutionary innovations. But there are several excellent reviews already out there, so another would be redundant. I’ll mention only a couple of points I think worth emphasizing below the fold.
First, robustness is a constant theme through the book. Robustness is defined as “the persistence of life’s features in the face of change.” Phenotypes are often invariant in the face of genetic change. There are multiple ways–indeed, very, very many ways–of metabolizing a given food source. Many different chemical reactions can metabolize a given food source. The chemical reactions that allow an organism to metabolize a given food source differ in detail within and across species, but are phenotypically the same. Note the “within”: there is variability within a species in how many food sources are metabolized: standing variation.
Second, genotypes are linked together in high-dimensioned spaces forming a massively interconnected network. Consider a large set of metabolic reactions, say N of them. Each reaction is a node in an N-dimensioned space. Each node has N-1 nearest neighbors, neighbors that differ in just one component of the reaction. Wagner’s interest is in the characteristics of that network. He finds that they are massively interconnected; that one can step from node to node without immediately or necessarily losing the metabolic properties of the ‘home’ node. A substantial proportion of the ‘neighbors’ in fact perform the same metabolic function: they have the same phenotype. And that goes for neighbors of the neighbors. Given roughly 5,000 total metabolic reactions known in all life, a space of 5,000 dimensions contains hundreds, perhaps even thousands, of phenotypically identical reactions as nearest neighbors of a given node, and hundreds more as nearest neighbors of that node’s neighbors, and so on. Wagner shows that one can step from node to node until the underlying genotype shares only 20% or 25% of its composition with the original ‘home’ node yet is phenotypically the same. That’s the source of robustness.
There’s more, of course, and my too-brief summary omits an enormous amount of detail. There are other implications for us. For example, those high-dimensioned interconnected networks with their phenotypically identical neighbors make nonsense of ID creationists’ probability calculations. Briefly, the notion that the numerator of their probability calculations is “1” is ludicrous. (Wagner refers to young-earth creationists as ““half literate and wholly ignorant.”)
I recommend the book heartily–it’s not only an excellent summary of Wagner’s ideas, but it’s also eminently readable. Here are a few links:
The book’s home site (Don’t get fooled by this creationist site.)
Barnes&Noble site (I read it on my Nook.)
The Amazon Borg site.
Mark Pagel’s review in Nature.
World Science Festival’s interview with Wagner.