Does evolution follow certain rules? For at least one important biological trait - body size - the answer is yes, new research suggests.
In one of the most comprehensive studies of body size evolution ever conducted, 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 of Stanford University.
The study reveals that over the past 542 million years, the mean size of marine animals has increased 150-fold.
The research also found that the increase in body size that has occurred since animals first appeared in the fossil record around 550 million years ago is not due to all animal lineages steadily growing bigger, but rather 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," he added.
Using photographs and detailed illustrations of fossils in the treatise, the researchers were able to calculate and analyse body size and volume for 17,208 marine genera.
A pattern soon became apparent: not all classes - groups of related species and genera - of animals trended towards larger size, but those that were bigger tended to become more diverse over time.
The scientists think this is due to advantages associated with a larger size, such as the ability to move faster, burrow more deeply and efficiently in sediment or capture larger prey.
"The discovery that body size often does evolve in a directional way makes it at least worth asking whether we're going to find directionality in other traits if we measure them carefully and systematically," Payne said.
The findings were published in Science.
