As an addendum to my last post, which referred to Kirschner and Gerhart’s The Plausibility of Life, I think it appropriate to visit the central tenet of the book. You see, this particular book looks at the incompleteness of the modern evolutionary synthesis in explaining the sources of variation, and proposes a corollary: “facilitited variation.” K&G outline this thesis in great detail, and I found the three properties that they enumerate (molecular mechanisms “descent with modification” throughout life’s history) particularly interesting. As they describe (page 223, below the fold):
- Weak linkage, a property particularly of signal transduction and transcription. In weak linkages, the protein interactions are weak and indirect. The signal is minimally informative and not instructional, whereas the response is maximally prepared and ready to be triggered. Weak linkage usually implies that a preconditioned response, which is self-inhibited, is released by the signal. With this prior preparation of responding components, demands on the signal for precision of interaction are low. Weak linkage facilitates the evolutionary change of signals and combinations.
- Exploratory behavior, a property of the processes forming the cytoskeleton, of processes operating in cell groups in development, and of functioning populations of organisms. The exploratory process has the capacity to generate an unlimited number of outcome states. Then, in response to an input, one or a few outputs are selected from among those states and retained, often by stabilization. Since only one state is eventually used, the unselected states are nonfunctional under the specific conditions. Yet these nonfunctional states may gain roles in future evolution. The selective agent, in its regulatory role, does not have to inform the process of its outcome. The process is built to be receptive to the agent, which simply serves as a stabilizing force, selecting one state among the large number of states generated in each instance. Therefore, new selectors are easily originated and with them new outcomes.
- Compartmentation, a property of embryonic spatial organization and cell type control. Compartmentation involves the use of weak linkage in the spatial dimension to specialize the behavior of different genes and different processes in different topological domains in the embryo. The domains allow a largely independent evolution of different regions of the animal, a property that has facilitated a large increace in the complexity of anatomy and physiology of animals without a corresponding increase in the complexity of the conserved core [cell and developmental] processes.
Well, I didn’t say it was an easy read to the layperson – but it is a reasonable outline of three memes made popular by work in recent decades in cell, developmental and molecular biology. Of personal interest, I noted that their “exploratory behavior” invokes elements of cell migration.
But the real point of this section is to outline the mechanisms by which phenotypic variation is generated for adaptive selection to act upon – the principle weakness of the modern evolutionary synthesis – and I think that they did a fair job. In the coming month or two, I plan to cover several more of similar treatises, using this book as a benchmark, and comment on them in this blog.