I.1.4.C Cope’s rule

Edward D.Cope, an American paleontologist proposed two laws in evolutionary biology

  1. Population lineages tend to increase body size over geological time.
  2. Animals which seem to be less specialised are found for a much longer time in the fossil record.

It is sometimes also known as the Cope-Depéret rule, because Charles Depéret explicitly advocated the idea that population lineages tend to increase in body size over evolutionary time.

Based on studies among fauna in North America, especially mammals, Cope proposed that the earliest fossil primates are very tiny and the later ones are large.

Large size enhances ability to avoid or fight off predators and capture prey, to reproduce, to kill competitors, to survive temporary lean times, and to resist rapid climatic changes. They may also potentially benefit from better thermal efficiency, increased intelligence, and a longer lifespan.

Offsetting these advantages, larger organisms require more food and water, and shift from r to K-selection. Their longer generation time means a longer period of reliance on the mother, and on a macroevolutionary scale restricts the clade’s ability to evolve rapidly in response to changing environments.

Examples:

  1. Eocene ancestors of modern horse were about the size of a dog.
  2. Camel, other herbivorous mammals, turtles, crocodiles, dinosaurs stand as an example to Cope’s rule.
  3. Cope recognised that clades of Cenozoic mammals appeared to originate as small individuals, and that body mass increased through a clade’s history.
  4. 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.

Exceptions:

The tendency towards size increase hasn’t been universal. There are many exceptions.

  1. Herbs and shrubs of recent times have originated from trees and other large plants.
  2. In planktonic Foraminifera, A.J.Arnold observed that newer species are of small size.
  3. Hooijer pointed out progressive size decrease in many vertebrates during Quarternary period.

Criticism:

It is only a descriptive generalization , does not apply to all living organisms. Hence anthropologists reject Cope’s rule as not being a law of nature.

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