Cope’s Rule

Cope’s rule is named after American palaeontologist Edward Drinker Cope. This rule states that an organism tends to increase in body size over time in a lineage of the population; for example, the ancestors of modern horses were the size of dogs. Evolution drives an increase in body size of the organism through time which can be seen in the fossil record. This rule has been demonstrated in many examples, but it doesn’t hold at all levels. Larger body size is related to increased fitness, but still, there are disadvantages like more prominent individuals are more vulnerable to extinction. This article will explain Cope’s rule meaning and provide you with Cope’s rule notes to understand this rule correctly.

Meaning of Cope’s rule

Cope’s rule, named after American palaeontologist Edward Drinker Cope, states that the body size of the population lineage increases over time. An organism tends to increase in body size over time. It suggests that there are extensive net selective advantages to being oversized. This rule shows that larger species usually control more energy locally and, in turn, maintain larger populations than expected, implying that larger organisms are better at acquiring and using resources. With dietary groups containing only small species, larger species do better. Large organisms find it easy to fight off predators, capture their prey, and kill competitors.

But on the other hand, larger organisms require more energy in terms of more food and water. Cope’s rule suggests that most animals have evolved towards bigger body sizes cannot be explained by the inherent advantages of a bigger size. A phylogenetic analysis of body size evolution in mammals shows that the size of the body increases remarkably in most inclusive clades.

Functions of the Cope’s Rule

Edward Cope and American palaeontologists stated two laws 

• Organisms tend to increase in body size over time 

• Less specialised animals are found in the fossil record for much longer.

Based on studies, especially mammals, Cope proposed that the earliest primate fossils were very tiny, and the later ones were large. The large size of these organisms enhanced their ability to fight off their predators and capture their prey. Examples are- Camels, turtles, crocodiles, dinosaurs. Coral sand dinosaurs seem to follow cope’s rule but birds and insects do not. Because of this, some scientists have wondered whether the pattern observed in mammals on land is a natural evolution phenomenon or a statistical one developing from nonselective and random evolution. The studies found that the average size of an animal today is around 150 times larger in size compared to half a billion years ago. So a question arises, why more significant? The researchers say that it is easier to eat other animals if you are large and avoid being eaten. Large animals have a higher metabolic rate, contributing to a more active lifestyle.

Validity of Cope’s Rule

Cope’s rule, which states that the size of an organism increases in lineages over time, is accepted by a vast majority of evolutionary ecologists. But the validity of this rule has been questioned lately. Backers of cope’s rule have been searching for mechanisms to explain the obvious trend (Hallam, 1975, Kingsolver and Pfennig,2004). Still, the opposition is taking a wider look and discovering biases present in earlier studies. Though Hallam’s work shows a strong trend towards size increase, it somehow is outdated compared to later studies.

In some cases, the increase in body sizes may act as a passive, not active, trend. In some cases, an increase in body size may represent conversion to optimal body size, and it does not necessarily mean the population develops to a larger size. Some experts are sceptical of Cope’s rule validity because, according to them, it only represents a statistical artefact. Examples to counter Cope’s rule are common; though size increase occurs more often than not, it is still not universal. An example is Cretaceous molluscs genera, an increase in body size is not more common than a decrease in body size. In most cases, Cope’s rule only works at a certain anatomic level, or it may work to some clades of a taxon. Cope’s rule is also supported in many examples, especially in marine invertebrate phyla, except molluscs. 

Conclusion

From a deep-time perspective, the tendency of clades to show an increase in size over evolutionary time is called Cope’s rule. Despite being in history for a long time, the mechanics and generality behind this rule is still a matter of debate. Palaeontologist Norman Newall started 70 years ago that Cope’s rule doesn’t apply to all the groups of organisms; he also noted that selection might not be limited to the whole individual in colonial organisms. He stated that there might be selected for increasing size in the parent individual and sexual generation. So this article is made to make you understand cope’s rule meaning and the