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In the Light of Evolution, Volume I: Adaptation and Complex Design
Each polymer is unstable and, after a short time, by chance, shrinks back from the tip (Kirschner and Mitchison, 1986). They probe all regions of the cell in a futile cycle of outgrowth and shrinkage. If one by chance encounters a stabilizing agent at the cell periphery, its end is trapped, preventing shrinkage (the selection event). The entire length of microtubule leading to the agent is preserved. As more microtubules are selectively stabilized in one location, the cell’s anatomy becomes polarized. This process is very adaptable and robust, providing microtubules no matter where stabilizers are located. It can therefore accommodate to placement errors or changing needs of the cell and can serve diverse roles, as in cilia, axons, and the mitotic spindle. Although the process of outgrowth and shrinkage is strongly conserved, and hence internally constrained in its own change, it generates diverse arrays each time it is used. In any particular cell, most outcomes are wasted, but they can be put to new uses in evolution simply by other cells’ placing selective agents in new locations.
Wiring of the nervous system also draws heavily on exploratory processes. Excess axons extend from the central nervous system and randomly explore the body’s periphery. Some accidentally hit target organs, such as muscles, and receive a dose of stabilizing protein (nerve growth factor); they persist, while others, failing contact, shrink back to the central nervous system.
ROBUSTNESS AND ADAPTABILITY
Weak regulatory linkage, state selection, and exploratory behavior underlie the robustness and adaptability of conserved core processes, that is, their capacity to produce functional (viable) outcomes despite physiological, developmental, environmental, or even evolutionary change. Robustness implies that a process remains the same because of tolerance or resistance to changing conditions, and adaptability implies that a process changes with the conditions in ways still to achieve the objective. Related to such properties, several authors have discussed the positive role of phenotypic plasticity in evolution (Schlichting and Pigliucchi, 1998; West-Eberhard, 2003); we feel that plasticity reflects the robustness and adaptability of core processes linked in complex assemblies. Robustness and adaptability are essential to the kind of evolution we have described, wherein core processes are used in different combinations, amounts, and states to produce new traits. They strongly reduce the requirements for regulatory change, and hence genetic change, and increase the frequency of viable phenotypic variations.
Adaptable robust processes can support nonlethal phenotypic variation in other processes, a situation called “accommodation” by West-Eberhard (2003). A specific example is the evolution of the tetrapod forelimb to a