In 1871, the American palaeontologist Edward Drinker Cope (1840–97) noted a phylogenetic trend towards increased body size in many animal groups, including mammals, reptiles, arthropods, and molluscs. This came to be known as Cope's rule. It remained unchallenged until a study of more than 1000 insect species in 1996 and was finally disproved in 1997, by a study in which David Jablonski made more than 6000 measurements on 1086 species of Late Cretaceous fossil molluscs spanning 16 million years and found that as many lines led to decreased size as to increased size. Evolutionary lineages show no overall tendency to greater size, but if the extant survivor happens to be larger than its immediate ancestor (e.g. the horse) this coincidence appears to validate Cope's rule. Cope also formulated a second rule, that species characterized by larger body size are more likely to go extinct.
Subjects: Ecology and Conservation.