PUNCTUATED EQUILIBRIA AND STASIS: A HIERARCHICAL, MACROEVOLUTIONARY PERSPECTIVE

Eldredge and Gould’s development of punctuated equilibria (PE) established invertebrate paleontology as a key discipline within evolutionary biology. Further, PE made macroevolution a key aspect of evolutionary studies concerned with pattern and process. One way this occurred was by properly re-centering species in evolutionary theory after a long period of mis-understanding what they represent. Key was reifying allopatric speciation from Mayr with evolutionary rates varying significantly through time from Simpson. Identifying stasis over millions of years was also a significant expansion to the synthesis. Many misunderstandings about PE revolve around inability to understand what species represent; misunderstandings of mechanisms of stasis arise for similar reasons. Another important development, led by Eldredge, involved hierarchies. Species typically comprise multiple populations, yet stabilizing selection, a population level process, is at times invoked as an explanation for stasis. What would be required for this mechanism to operate? Either a species would be a single population for millions of years, or, if multiple populations, the selection pressures in the different communities those populations occupied would be equivalent. Further, constant selection pressures would have to prevail over millions of years, requiring constant environments. These situations prevail rarely if ever. Stabilizing selection can only maintain stasis if it operates on the specific mate recognition system, a view recognizing the hierarchical nature of species. A more tractable, better supported mechanism for stasis invokes the geographic and hierarchical structure of species. Multiple populations experience different selective pressures; if they continually reaggregate this will homogenize change, leading to net stasis. Stasis would not be obdurate but oscillatory. To better understand the causes of stasis it is necessary to focus on processes spanning several hierarchical levels. To the extent that aspects of the genome and development are constrained, mechanisms of stasis are likely to derive from lower levels. To the extent they are not, they are likely to derive from higher levels, especially the species level, since population level phenomena are unlikely to contribute much to stasis.