Seven years ago, I was sitting outside a hotel in China waiting for lunch to start (yes, I worked at gaining weight back then), when Caltech developmental biologist Eric Davidson walked up and asked me why I was carrying a diagram from one of his papers. The diagram depicted the complex control region of the Endo16 gene in sea urchins. I told him that I wanted to ask conference participants what process they thought had constructed the highly specified genetic circuitry (over a dozen DNA-binding proteins, interacting at nearly three dozen binding sites, to construct the sea urchin gut) -- what Davidson described as "information processing units 'wired' into the regulatory network so that they receive multiple inputs" (2001, p. 7).
Davidson smiled, somewhat ruefully, and said, "Well, I'm not sure, but I know that standard single-base-pair mutations won't do it" -- meaning, as he later explained to me, the textbook neo-Darwinism every college biology student learns. He was more blunt with the science writer Fred Heeren, who was covering the now-notorious conference we were attending. "Neo-Darwinism is dead," he said in an interview.
Davidson brought his case for the insufficiency of standard evolutionary theory to the pages of Science this past week. Writing with the paleontologist Doug Erwin, he argued that the "establishment by the Early Cambrian of virtually all phylum-level body plans" is not explained by the usually-invoked evolutionary processes:
Classic evolutionary theory, based on selection of small incremental changes, has sought explanations by extrapolation from observed patterns of adaptation. Macroevolutionary theories have largely invoked multi-level selection, among species and among clades. But neither class of explanation provides an explanation of evolution in terms of mechanistic changes in the genetic regulatory program for development of the body plan, where it must lie. (p. 796)
Later in the same paper:
Current microevolutionary thinking assumes that observed types of genetic change (from single base substitutions to gene duplications) are sufficient to explain all evolutionary events, past and present....But attempting to explain an aspect of animal evolution that depends on one kind of network alteration [deep changes] by adducing evidence from an aspect that depends on another [shallow changes] can be fundamentally misleading. (p. 800)
As skeptics of neo-Darwinism have long observed, showing that pigmentation varies in moth wings does not explain how moths themselves evolved. Starting with this insight, Davidson and Erwin classify the genetic instructions responsible for evolutionary change into what they call "kernels," "plug-ins," and "differentiation gene batteries."
Operating at varying degrees of depth [or functional importance] within the developmental hierarchy, these genetic modules affect, respectively, body plan [kernel] characters; class, order, and family characters [plug-ins]; and species characters [differentiation gene batteries]. And Davidson and Erwin make a bold prediction.
Kernels -- the deepest levels of developmental control -- cannot vary. "[I]nterference with expression of any one kernel gene will destroy kernel function altogether and is likely to produce the catastrophic phenotype of lack of the body part" (p. 796). As they explain,
We think that change in them is prohibited on pain of developmental catastrophe, both because of their internal recursive wiring and because of their roles high in the developmental network hierarchy. We predict that when sufficient comparative network data are available, there will be found conserved network kernels...which program the initial stages of development of every phylum-specific body part and perhaps of superphylum and pan-bilaterian body parts as well. (p. 799)
If changing the wiring takes down the whole system, well, then, obviously the wiring can't change -- a developmental instance of what has come to be known as the Principle of Continuity. Like I said, it's a bold prediction, worthy of praise. [Fear of being wrong is the bane of science, because if everybody clutched their gonads all day, trying to find the warm center of the herd, nothing would get done.]
While a graduate student, I observed firsthand as a similar prediction was formulated for Drosophila and the arthropods. The prediction crashed and burned -- but that's a story for tomorrow, kids.
Reference
Eric Davidson, Genomic Regulatory Systems: Development and Evolution (New York: Academic Press, 2001).
