USING THE EXTENDED PRICE EQUATION TO ANALYZE SPECIES SELECTION IN MAMMALIAN BODY SIZE EVOLUTION ACROSS THE PALEOCENE/EOCENE THERMAL MAXIMUM

Species selection is an important component of macroevolution. If a particular trait varies between species, and that variation leads to differential extinction and/or speciation, then some species will diversify while others vanish. Despite its significance, much of the research on species selection has relied on indirect evidence to demonstrate its presence and few studies have been able to determine its contribution to evolution. Using a novel extension of the Price equation, we identify and quantify the influence of species selection, as well as immigration and anagenetic change, on the evolution of the mean body size of mammals from the Bighorn and Clarks Fork basins of Wyoming across the Paleocene/Eocene boundary (approximately 56 million years ago). This boundary is closely associated with the Paleocene/Eocene Thermal Maximum, during which temperatures may have increased by as much as 5 to 9°C across a span of just 100 000 years, and the apparent dwarfing of numerous mammalian taxa compared with their stratigraphically older and/or younger relatives. In agreement with other studies, our results document the remarkable decrease in mean mammalian body size across the boundary, a pattern that is shown to be principally attributable to the immigration of small species and, to a lesser extent, anagenetic evolution towards smaller body sizes. Species selection across the same interval, intriguingly, favored larger-bodied mammalian species, suggesting that natural selection and species selection need not favor the same trait. As interest in the response of mammals to dramatic climatic events has increased more recently, the patterns of mammalian evolution across the Paleocene-Eocene Thermal Maximum have become an important focus of research. If our findings are any indication, species selection might be more influential during warming events than previously recognized.