The Role of Development in Evolutionary Radiations

How are biodiversity dynamics developmentally constrained at various hierarchical levels? Patterns of diversity and associated process theories have conventionally been treated in extrinsic, particularly ecological, terms, and development has not been sufficiently integrated into discussions of diversity change (e.g., Rosenzweig 1995). This may be due, in part, to a reluctance of many students of development or ecology to delve into the interplay between allegedly ahistorical principles of form generation and their historical realization in the process of evolution. It was not until recently, when the reality of the Cambrian explosion of metazoan body plans became clear, that developmental explanations have been called on to account for the apparent temporal asymmetry in the turnover of higher taxa (and associated morphological innovations). Changes in developmental integration and regulation are now an interesting alternative hypothesis to the usual ecological explanations (e.g., Erwin and Valentine 1984; Jablonski and Bottjer 1990a; Hall 1992; Erwin 1993, 1994; Valentine 1995; Raff 1996). An array of macroevolutionary problems associated with changes in diversity through time has not yet been fully explored in explicitly developmental terms. Is diversification at lower levels ever developmentally constrained? What is the developmental context of key innovations? Can developmental constraints underlie failure to radiate and macroevolutionary lags (low-diversity delays to diversification -- Jablonski and Bottjer 1990b)? Are diversity-dependent phenomena always played out in ecospace? Null hypotheses for diversification have usually been based on the assumption of randomness, but given nonrandom pattern we must logically pose development-based explanations as alternative hypotheses to ecology-based ones. Testing such explanations is a basic challenge faced by neontologists and paleobiologists alike. I address the role of development in the context of evolutionary radiations, not only because they have traditionally attracted the attention of both paleobiologists and neontologists, but also because problems of testability become more evident. However, much of the discussion is potentially applicable to background diversification times as well. Additionally, I present a case study on a recent hypothesis for the early introduction of evolutionary innovations (as in the Cambrian explosion) that purportedly includes contributions from both ecology and development: Kauffman's model of rugged fitness landscapes (Kauffman 1989, 1993, 1995). Appears as "The role of development in evolutionary radiations," in Biodiversity Dynamics: Turnover of Populations, Taxa, and Communities,edited by M. L. McKinney. New York: Columbia University Press (1998), pp 132-161.