Phenomenological Levels and Evolutionary Rates

Abstract

Eldredge, N. (Department of Invertebrates, The American Museum of Natural History, Central Park West at 79th St., New York, New York 10024) 1982. Phenomenological levels and evolutionary rates. Syst. Zool., 31:338–347.–Traditionally, taxonomic rates of evolution have served primarily as estimators of “transformational” rates—i.e., rates of anatomical and underlying genetic change. This approach comes from the Modern Synthesis, which sees all evolutionary patterns reducible (at least in principle) to simple generation-by-generation change in gene content and frequency. If, however, species are individuals (sensu Ghiselin, 1974) rates of speciation (and species extinction) are important intrinsically, and cannot be construed as a mere function of rates of transformational change. In general, it can be argued that for every kind of biological individual posited, there is implied a quasi-discrete phenomenological level of the evolutionary process. Different sorts of biological individuals are nested hierarchically (for example, individual organisms within species), conveying an ontological (rather than purely epistemological, cf. Stebbins and Ayala, 1981) basis to the claim that evolution involves more than one phenomenological level. Each level has its characteristic patterns of change produced by its own array of deterministic and stochastic processes. Such processes cause a shuffling of component smaller individuals within the next higher individual. Rates of change within the levels are to a significant extent independent of one another. Five levels of the evolutionary process are briefly characterized and discussed. [Evolutionary rates; macroevolution; speciation.]