Cope’s rule, named after American paleontologist Edward Drinker Cope, postulates that population lineages tend to increase in body size over evolutionary time.
According to Cope’s Rule, only those animals which can adapt to any type of environment alone can survive and they become larger and larger in course of evolution. The fossil record appears to shows that animals in the same lineage get larger as they evolve. The earliest fossil primates were very tiny but the later ones were larger.
Cope cited several examples in favor of his rule. Horse, camel and all other herbivorous mammals during their course of evolution exhibits in increase in size and some of them even attained gigantism. Organisms other than mammals such as turtles , crocodiles etc. also exhibits the same phenomenon. Dinosaurs provide the most interesting example of attaining gigantism.
There are many objections to Cope’s Rule. Cope’s observations were based upon mammals but a number of mammalian lineages like insectivores in general and insectivorous bats in particular do not show the tendency towards increase in size. It is physically impossible for a mole to be very large, because that will create difficulty in burrowing. Similarly, the flying habit imposes limitations on body size. Many mammalian lineages exhibit decrease in size. For example, Ungulates and carnivores attained gigantism in Pliocene and Pleistocene but at present they are not gigantic.
However, it is now presumed that increase in size in different groups of mammals may be on account of different reasons. In Pleistocene mammals , it could be a device to reduce the ratio of body surface in relation to body volume to conserve heat in the cooling environment .In carnivorous animals increase in size could facilitate the capture of large sized herbivores on them they fed. In egg laying animals this could be a device to increase the fecundity i.e. more eggs can be accommodated in a large body
Evolution scholars rejected this law as it is not applicable to living organisms and just descriptive generalization