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In the Light of Evolution, Volume I: Adaptation and Complex Design
bird or bat wing. Not only did the length and thickness of bones change, but also the associated musculature, nerve connections, and vasculature. Did many regulatory changes occur in parallel, coordinated by selection, to achieve the coevolution of all these tissues in the limb evolving to a wing? The answer comes from studies of limb development showing that muscle, nerve, and vascular founder cells originate in the embryonic trunk and migrate into the developing limb bud, which initially contains only bone and dermis precursors. Muscle precursors are adaptable; they receive signals from developing dermis and bone (Kardon et al., 2003) and take positions relative to them, wherever they are. Then, as noted previously, axons in large numbers extend into the bud from the nerve cord; some fortuitously contact muscle targets and are stabilized, and the rest shrink back. Finally, vascular progenitors enter. Wherever limb cells are hypoxic, they secrete signals that trigger nearby blood vessels to grow into their vicinity (Ferrara et al., 2003). This self-regulating vasculogenesis operates not just in the limb but throughout the body, accommodating to growing tissues, to exceptional demands such as pregnancy, and alas to growing tumors. The adaptability and robustness of normal muscle, nerve, and vascular development have significant implications for evolution, for these processes accommodate to evolutionary change as well. In the case of the evolving wing, if bones undergo regulatory change (driven by genetic change) in length and thickness, the muscles, nerves and vasculature will accommodate to those changes without requiring independent regulatory change. Coevolution is avoided. Selection does not have to coordinate multiple independently varying parts. Hence, less genetic change is needed, lethality is reduced, larger phenotypic changes are viable, and phenotypic variation is facilitated.
Finally, as Schmalhausen, Waddington, and others (Waddington, 1953; Schmalhausen, 1986; Gibson and Dworkin, 2004) have argued, physiological and developmental robustness reduces lethality because of undirected genetic variation. Less genetic variation is eliminated from the population, leaving it available for new trials of regulatory combinations and effects.
FAVORABLE SOURCES AND PATHS OF PHENOTYPIC CHANGE
Several authors tried in the past to connect long-term evolutionary change to short-term physiological change. As well known, Lamarck postulated that animals undergo anatomical and physiological changes in response to the environment, and then their offspring inherit these acquired characteristics. Darwin first conceived of variation as undirected and small with respect to selective conditions but later drifted toward Lamarck in thinking that as the organism responds to conditions, it furnishes the gametes with information enhancing the next generation’s response. In