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and alternate jaw structures. In her view, if the capacity to develop large phenotypic differences already exists in the organism as self-inhibited alternate states, and these can be elicited by simple signals (weak linkage), then large evolutionary steps can be made with a modicum of genetic change. In such cases, the distinction blurs between evolutionary gradualism and saltation (the generation of significant traits by single mutations). As an example, sex in some vertebrates (fish and reptiles) is determined environmentally (temperature, crowding, or social interactions) but in others, heritably (sex chromosomes). The underlying mechanisms for sex determination are similar in all vertebrates. It is just that an environmental stimulus (acting via weak linkage) has been replaced by a genetic one in the sex chromosome case. Neither provides much information about the outcome but just acts on the conserved switch.

To summarize, the relevant point of these examples is that regulatory change is easily effected when conserved core processes have an inherent capacity for weak regulatory linkage, that is, when switch-like behavior and alternative states of function are already built into them. The regulator need not inform the response or be stereochemically compatible with it. Regulation does not need to coevolve with the functional response. The requirements for regulation and regulatory change are reduced.

EXPLORATORY PROCESSES

As the name implies, some conserved core processes appear to search and find targets in large spaces or molecular populations. Specific connections are eventually made between the source and target. These processes display great robustness and adaptability and, we think, have been very important in the evolution of complex animal anatomy and physiology. Examples include the formation of microtubule structures, the connecting of axons and target organs in development, synapse elimination, muscle patterning, vasculogenesis, vertebrate adaptive immunity, and even behavioral strategies like ant foraging. All are based on physiological variation and selection. In the variation step, the core process generates not just two output states, but an enormous number, often at random and at great energetic expense. In the selective step, separate agents stabilize one or a few outputs, and the rest disappear. Although that agent seems to signal the distant process to direct outputs to it, it actually only selects locally via weak linkage among the many outputs independently generated by the process. Components of the variation and selection steps of the process are highly conserved.

Microtubules, for example, adopt vastly different spatial arrays in different cells. First, the tips of numerous microtubule polymers grow outward from a nucleation center, in random directions (the variation event).



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