Measuring the evolution of body size in mammals.

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In Tempo and Mode of Evolution, an early classic of the evolutionary synthesis, George Simpson argued that, although many aspects of evolution can be studied in living populations, rates and patterns of evolutionary change are best documented with the fossil record (1). The fact that fossils can be placed into phylogenetic and stratigraphic frameworks makes it possible to calculate rates of change directly rather than extrapolating them from the inferred history of living organisms. Although Simpson's logic is sound in principle, the task of measuring evolutionary rates from fossils is laborious in practice: Fossils must be taxonomically assigned to species and then assembled into a phylogenetic tree by using methods that distinguish true ancestors from sister groups; the sediments in which fossils are found must be correlated lithologically or biostratigraphically and then packages of rocks must be correlated into a global timescale or otherwise given absolute ages; and finally, the numbers of generations represented by the intervals of time separating the fossils must be estimated (2). Because of the herculean task of gathering these basic data, most paleontological studies of evolutionary rates are narrow in scope, confined to a few lineages or clades, too piecemeal to say much about the broad-scale patterns that are normally the strength of the paleontological record. In PNAS, Evans et al. (3) develop a “shortcut” that allows evolutionary rates to be compared across all mammals over the entire Cenozoic Era (last 65 My), giving a very broad picture indeed. They find some unexpected things, including an evolutionary bias in which body mass appears to decrease faster than it increases. Importantly, their paper puts numbers on the amount of evolutionary time required for body mass changes of different magnitudes.