INVENTION, INNOVATION AND ERGODICITY: AN ALTERNATIVE APPROACH TO MACROEVOLUTION

Modern macro-evolutionary theory has emphasized a hierarchical expansion of evolution, with sorting and selection at multiple levels, from cell lineages to populations, species and clades. This approach contrasts with the original form of macroevolution that was principally concerned with mutationally-driven novelties as a source of maroevolutionary innovation. Like virtually all modern views of evolution, these views are ergodic or uniformitarian: they assume that fundamental, underlying structure of evolution is constant and timeless. Modern physics is ergodic, assuming that the laws of quantum mechanics apply uniformly through space and time, and economics makes similar assumptions (albeit, as economist Douglass North has observed, with far less justification). Yet evolution, like economics, is a fundamentally historical discipline in which the structure changes through time. Major evolutionary transitions including the origin of the genetic code, construction of the eukaryotic cell, the repeated advents of multicellularity and the Ediacaran-Cambrian radiation of bilaterian animals were historically unique events that constrained the nature of subsequent variation and also permitted (although did not require) the construction of new ecological relationships (e.g. trophic feeding levels). Major evolutionary transitions thus form a nested set of events that change patterns of available variation, limiting the scope of classical population genetics. The Ediacaran-Cambrian radiation of bilaterians provides paradigmatic examples of these issues. Recent work with Eric Davidson (CalTech) has documented the existence of highly conserved genetic linkages at the core of specific developmental circuits, which we have termed kernels. Once formed, these kernels appear to be highly resistent to further modification, although upstream inputs and downstream outputs may be (and are) frequently alterned. Kernels are easily distinguished from developmental ‘plug-ins' that are components of a frequently utilized genetic toolkit. The construction of a myrid of new ecological relationships during the initial radiation of Bilateria similarly changed the structure of the evolutionary theater, both creating new opportunities and limiting others.