Cope's Rule

Cope's Rule—that famous 19th-century notion that there is a general tendency toward size increase in evolution (J. Alroy, Reports, 1 May, p. 731)–has become a fixture of debates about pattern and process in paleobiology. Many recent studies have concluded that trends toward increased size are illusory, although some confirmed them in specific groups (1–3). Other papers have commented on possible mechanisms explaining Cope's Rule: some argued for co-adaptation, some for species sorting, and some for context-dependent statistical factors (4, 5). All used new data or new logic to evaluate Cope's long-held truism, which has arguably dominated our perception of the fossil record for more than a century.