Re: ORIGINS: Phyletic Change

David Campbell (bivalve@isis.unc.edu)
Fri, 13 Sep 1996 11:45:14 -0500

Although the pattern of higher taxonomic categories appearing earlier
certainly exists, caveats about the data are important. There is no
absolute definition of a genus or any higher taxon, and many taxa become
"extinct" by evolving into something else. For example, Art Boucot (who
generated much of the data) has pointed out that a brachiopod "extinction"
in the mid-Paleozoic that appears in Sepkoski's early data sets is simply a
taxonomic artifact and not an actual extinction. If the last reviser of
late Cambrian trilobites was a splitter and the last reviser of early
Ordovician trilobites was a lumper, a compilation of data without taxonomic
review would appear to have a big end-Cambrian extinction.
Certainly, a purely gradualistic model of evolution would not predict that
all phyla (or at least all those with a reasonable record) appeared early,
but many would be expected to appear early. Early branches would have the
most time to evolve into very different modern forms. Under a model of
punctuated equilibrium, most if not all higher categories could appear
early.
By definition, a higher taxonomic category must appear before or at the
same time as the oldest lower category it contains. This alone would
produce a greater average age of higher categories than lower, especially
when most higher categories contain several of the next lower category.

Department of Geology
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug