What is Cope's Rule?

Cope’s rule, the gradual trend towards an increase in body size over time, has long been a source of controversy in evolutionary ecology. Edward Drinker Cope was a palaeontologist who described the first important faunas from North America (Alroy, 1998). While working with these specimens he discovered that there was an increase in the average size of mammals during the Cenozoic time period (Alroy, 1998). He came to the conclusion that this pattern must have been the result of a tendency for new groups to evolve at small sizes and a continuous push towards a larger body size (Alroy, 1998). Since the time of Cope, studies have been performed for many taxa with differing results. The debate focuses mainly on whether this macroevolutionary process actually exists and if it does, what are the microevolutionary processes that drive it.

Figure. Above left is a late Miocene pelecypod. On the right are two Pliocene pelecypods. The question remains as to whether there was evolution towards a larger body size.