“TEMPO AND MODE” OF EVOLUTION RECONSIDERED, AGAIN: WHAT IS THE RELATIVE FREQUENCY OF DIFFERENT MACROEVOLUTIONARY DYNAMICS IN THE FOSSIL RECORD?

The terms by which we call different patterns of evolution are not just labels; they reflect and affect the way we think about the causes of those patterns. In 1944 G.G. Simpson suggested that in studying evolution, there are “two topics in particular” on which “the paleontologist enjoys special advantages”: first, estimation of evolutionary rates (which he called “tempo”), and second, “the way, manner, or pattern of evolution” (which he called “mode”). He described tempos as varying from very slow to very rapid, and the “three major styles, the basic modes of evolution” as “speciation, phyletic evolution, and quantum evolution”. Gould and Eldredge (1977) restricted “mode” to phyletic evolution (anagenesis) and speciation (cladogenesis), and claimed that mode could be inferred “from the empirical distribution of tempos”. Punctuated equilibrium (PE), they said, “is a hypothesis about mode”. The recent literature often takes a looser approach to these terms. Evolutionary “modes” now regularly refer to 3 patterns of anagenesis: directional, random walk, and stasis, and analyses of the relative frequency of these are taken as evidence of the relative frequency of PE, and/or other macroevolutionary patterns.

The several compilations of “tempo and mode” from the fossil record are mostly of anagenesis, which can be used to infer mode (e.g., dominance of stasis implies punctuated change). PE, however, requires persistence of ancestors to distinguish it. Fossil morphospecies are not necessarily equivalent to species recognized in the Recent, but can still be used for this test.

Preliminary analysis of a growing dataset of phylogenies, mostly of Cenozoic marine mollusks, which allow for recognition of persistence of putative ancestral species, suggests that anagenetic transformation is frequent, but stasis is more so. Demonstrable persistence of ancestors occurs in less than half of all lineages, indicating that while it does occur, PE is not the dominant pattern.