Speciation Rates and Morphological Divergence in Fishes: Tests of Gradual versus Rectangular Modes of Evolutionary Change.

Eldredge and Gould (1972)advanced an exciting hypothesis to account for the evolutionary diversification of life. They propose that the great majority of organismal divergence accompanies the splitting of lineages (cladogenesis), and that new species once formed represent well-buffered homeostatic systems, resistant to within-lineage change (anagenesis) until the next round of splitting occurs. Classical branching "trees of life," in which organismal divergence is plotted on one axis and time on another, are thus better represented as "rectangular" branching patterns (Stanley, 1975) reflecting evolution through "punctuated equilibria" (Fig. 1). If this view of evolution is correct, then gaps in the fossil record are real, a consequence of the rapid evolutionary change occurring perhaps in geographically localized populations undergoing speciation. In contrast, under models of "phyletic gradualism," these same gaps are interpreted as incomplete fossil transcripts of gradual evolutionary change. Thus, as noted by Eldredge and Gould (1972), "If we doubt phyletic gradualism, we should not seek to 'disprove' it 'in the rocks. '" Avise and Ayala (1975) introduced general models which examine the logical consequences of rectangular evolution versus phyletic gradualism for the distributions of genetic distances among living species. The rationale and approach are simple. Under rectangular models, since most change occurs during speciation, extant members of highly speciose phylads