Cope’s Rule holds that there is active selection for increasing body size in nature. The new study, published in the journal Science, finds this to be true. It appears that evolution follows certain rules and can, at least to some extent, be predicted.
“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,” co-author Jonathan Payne, a paleobiologist at Stanford’s School of Earth, Energy & Environmental Sciences, said in a press release.
This is not to say that huge animals didn’t existed in the past. The gigantic dinosaur Dreadnoughtus, for example, measured 85 feet long and weighed about 65 tons. The point is that, as many animal lineages evolve, there is a tendency toward larger sizes, particularly if food resources, the environment and other factors remain relatively stable.
The effect has been dramatic for animals like horses. As seen in the accompanying image, the earliest horses were much smaller than those alive today. Humans, in general, are also growing larger. In Britain, for example, the average height of a 21-year-old man increased from 5’5” in the 1870s to 5’10” as of the 1970s, according to researchers from the University of Essex and the National University in Canberra.
For the latest study, Payne and his team tested out Cope’s Rule on marine animals. The scientists compiled a dataset that included adult body size measurements for individual species within more than 17,000 groups, or genera, of marine animals spanning five major phyla: Arthropods, Brachiopods, Chordates, Echinoderms and Mollusks.
“Our study is the most comprehensive test of Cope’s Rule ever conducted,” said lead author Noel Heim of Stanford University. “Nearly 75 percent of all of marine genera in the fossil record and nearly 60 percent of all the animal genera that ever lived are included in our dataset.”
Analysis of the massive dataset revealed that, over the past 542 million years, the mean size 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 explained. “This may not seem like a lot, but it represents a big jump.”
The researchers additionally found that the increase in body size, which has occurred since animals first appeared in the fossil record around 550 million years ago, is not due to every animal lineage steadily growing bigger. It’s instead due to the diversification of groups of organisms that were already larger than other groups early in the history of animal evolution.
“That’s also something we didn’t know before,” Payne said. “For reasons that we don’t completely understand, the classes with large body size appear to be the ones that over time have become differentially more diverse.”
They suspect this is because of advantages associated with a larger size, such as the ability to move faster, to burrow more deeply and efficiently in sediment or to capture larger prey.
“It’s really a story of the survival and diversification of big things relative to small things,” Heim said.
Small things aren’t all staying small either. Yet another recent study found that certain spiders are growing bigger in urban areas.
There’s no need to fear monster-sized ants, termites or other such things in our lifetimes, though. Growth within a species — if it happens at all — occurs slowly and over many generations.
Read more at Discovery News
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