Animals tend to evolve toward larger size over time


Does evolution follow certain rules? If, in the words of the famed evolutionary biologist Stephen Jay Gould, one could “rewind the tape of life”, would certain biological trends reemerge? Asked another way: can evolution be predicted? New research suggests that, for at least one important biological trait-body size-the answer is yes.

In one of the most comprehensive studies of body size evolution ever conducted, Stanford scientists have found fresh support for Cope’s rule, a theory in biology that states that animal lineages tend to evolve toward larger sizes over time.

“We’ve known for some time now that the largest organisms alive today are larger than the largest organisms that were alive when life originated or even when animals first evolved,” said Jonathan Payne, a paleobiologist at Stanford’s School of Earth, Energy & Environmental Sciences. What was unclear, however, was whether the average size of animals has been changing over time and, if so, whether that reflects a trend, or directionality, in body size evolution. “It’s not something that you can know by just studying living organisms or extrapolating from what you see over short time scales. If you do that, you will absolutely be wrong about the rate, and possibly also the direction,” Payne said.

The study, published in the Feb. 20 issue of the journal Science, reveals that over the past 542 million years, the mean sized of marine animals has increased 150-fold. “That’s the size difference between a sea urchin that is about 2 inches long versus one that is nearly a foot long,” Heim said. “This may not seem like a lot, but it represents a big jump.” Edited from Animals tend to evolve toward larger size over time, study finds.

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