Cope's rule

In evolutionary biology, Cope's rule states that population lineages tend to increase body size over geological time. It is named for Edward Drinker Cope. The horse family, Equidae, is often used to illustrate the rule, with small animals evolving into larger ones; but critics such as Stephen Jay Gould point out a number of shortcomings of this example.

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Cope's rule is interesting because it implies the apparently (but not actually) paradoxical suggestion that possession of large body size favours the individual but renders the clade more susceptible to extinction.

Cope's rule, or the evolutionary trend toward larger body size, is common among mammals. Large size enhances the ability to avoid predators and capture prey, enhances reproductive success, and improves thermal efficiency. Moreover, in large carnivores, interspecific competition for food tends to be relatively intense, and bigger species tend to dominate and kill smaller competitors. Progenitors of hypercarnivorous lineages may have started as relatively small-bodied scavengers of large carcasses, similar to foxes and coyotes, with selection favoring both larger size and enhanced craniodental adaptations for meat eating. Moreover, the evolution of predator size is likely to be influenced by changes in prey size, and a significant trend toward larger size has been documented for large North American mammals, including both herbivores and carnivores, in the Cenozoic.
—(Science, Vol 30 doi:10.1126/science.1102417)

Cope's rule has come under sustained criticism, including the observation that counterexamples to Cope's rule are common throughout geological time, although size increase remains the rule rather than the exception. Critics also point out that the so-called rule is worthless without a mechanism.^{[citation needed]} Note that semantically the "rule" in this context (unproven assumption with exeptions) refers more to a rule of thumb, trend or a belief than to a truth, law, fact or a norm.