USING THE PRICE EQUATION TO QUANTIFY SPECIES SELECTION AND OTHER MACROEVOLUTIONARY FORCES IN CRETACEOUS GASTROPODS

Species selection has been a challenging concept to study in the context of macroevolutionary history. Previous work demonstrates the occurrence of, or prerequisites for, species selection, but does not quantify the strength of species selection relative to other macroevolutionary forces. Rankin et al. (2015) used the Price equation to quantify the effects of species selection, anagenetic change, and immigration on mean mammalian body size during the Paleocene/Eocene Thermal Maximum. Here, we use the same approach to study macroevolution of mean geographic range size in late Cretaceous marine gastropods (originally studied by Jablonski (1987)). The data cover sixteen million years before and during the end-Cretaceous mass extinction. Mean range size fluctuated by hundreds of square km over this period, increasing substantially immediately before and during the end-Cretaceous mass extinction. Using data on species' range sizes and their ancestor-descendant relationships, we found that species selection consistently favored species with larger geographic ranges, but was small in magnitude and consistent with random chance except immediately before the end-Cretaceous mass extinction when many small-ranged species went extinct. The effects of immigration and anagenetic change were comparable in mean magnitude to species selection, but varied over time in different ways. Immigration typically moderately reduced mean range size, but an unusual pulse of large-ranged immigrants early in the time series substantially increased both mean range size and the interspecific variance in range size. Increased variance in range size may have increased the scope for later species selection. Anagenetic change typically slightly increased mean range size, but drove an unusually large decrease in mean range size immediately preceding the strong species selection in the run-up to the end-Cretaceous mass extinction. Anagenetic change towards smaller range sizes thus set the stage for subsequent strong species selection against small-ranged species. The Price equation quantifies the effects of different macroevolutionary forces on a shared scale, and reveals the interplay of macroevolutionary events leading up to mass extinctions. These results can inform the search for underlying biological mechanisms.