Macroevolution: The morphological problem

Abstract

The central question of macroevolution concerns the evolution of major morphological innovations (and major taxonomic groups). It is a matter of scale rather than simply rate of evolution or hierarchical level of mechanism. Through the history of microevolutionary theory there is a constant counterpoint of macroevolutionary questioning: are current versions of microevolution sufficient to explain the data concerning origins of major novelties? Thus, Simpson proposed the term "megaevolution" and "quantum evolution." Mayr partially separated micro- and gradual aspects by proposing "genetic revolution" as a mechanism for rapid change. Evolutionary theory in general suffers because different concepts have incompatible frames of reference. Key innovation and correlated progression are concepts that approach the problem in terms of evolutionary morphology. They have in common with quantum evolution and genetic revolution features such as threshold effect, preadaptation and smooth transition due to change in function and environment. One aspect of paleontology and morphology, however, is to show that some morphologies can only exist in binary states, with no intermediates. This leads us to look away from selection on graded series of phenotypes to development (developmental cascade/threshold models) for new levels of explanation. © 1992 by the American Society of Zoologists.