The observed tendency for body size to increase during the evolution of a lineage is called Cope’s rule. It is named after American paleontologist Edward Drinker Cope, although Cope himself did not state the rule in its modern form.
The idea describes a broad evolutionary trend, not a universal law. Larger bodies can offer advantages such as improved defense against predators, greater ability to capture prey, or increased success in competition and mating. These benefits may create selection favoring larger individuals over many generations.
Size increase also has costs. Large organisms generally require more food and water, mature more slowly, and may be more vulnerable to extinction when environments change. Researchers have found examples supporting Cope’s rule in some mammal, dinosaur, marine-animal, and other lineages, but many exceptions exist. In some cases, apparent size increases may reflect statistical sampling or diversification patterns rather than a consistent push toward larger bodies. Dollo’s law concerns evolutionary irreversibility, while Bergmann’s rule concerns body size and climate.