Evolution Update

Evolution Update

Are Animals Evolving Towards a Larger Body Size?

Christopher Jenness February 24, 2015

A comprehensive analysis of the fossil record reexamines a 19th century assumption.

A speculation dating back to the 19th century in evolutionary biology is the idea that animals will evolve larger body sizes over evolutionary time. Named after the famous paleontologist Edward Cope, this concept is known as “Cope’s rule.”

Cope’s rule presents an interesting problem for evolution. Large body size creates a variety of evolutionary burdens. Larger animals take longer to generate offspring. For example, humans typically take around 20 years to have children while cats only take a few. Additionally, larger animals usually have smaller population sizes than smaller animals. These, and other aspects of large animals should not be favored in evolution, so it appears that evolution should trend towards smaller animal body size.

While seemingly disadvantageous, a lot of fossil evidence supports the idea that animals evolve toward larger body size. For example, when examining fossils in North America, it seems likely that mammals have evolved larger body sizes over time. Similarly, birds appear to follow Cope’s rule.

In contrast to birds and North American mammals, a growing amount of fossil evidence also contradicts Cope’s rule. For example, turtles have not strictly followed Cope’s rule in evolving towards larger body sizes. These discrepancies have left scientists to question the validity of Cope’s rule.

In order to gain insight into the extent that Cope’s rule holds up across a variety of animals, Stanford scientists went back and reexamined the fossil record. Specifically, they looked at over 17,000 varieties of marine animals over roughly the last 500 million years. This large scale analysis of the fossil record included nearly three quarters of the diversity present in the complete fossil record to date.

By measuring and estimating body size from marine fossils, scientists realize that the average body size increased by a factor of 150 over the last 500 million years. This evidence is in direct support of Cope’s rule. Breaking it down between large and small animals, the smallest animals did not change much over evolutionary time, however, the large animals evolved to much larger sizes.

Although previous studies have attempted to verify Cope’s rule using the fossil record, this study was truly remarkable in scope. The variety and quantity of fossils examined was so great that this study provides the strongest evidence to date that animals are evolving towards a larger body size.

Using sophisticated models, the authors conclude that evolution towards larger body size was not the result of random drift. Instead, it appeared to be an active process over the last 500 million years. This poses the immediate question: At what point will animal evolution towards larger body size max out? What is the upper limit on body size that animals can maintain? Cope’s rule likely cannot be sustained indefinitely and it will be interesting to hear where it is expected to plateau.

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