As species evolve, they get bigger -- or smaller

Jablonski's research disproves belief that organisms always increase in size Paleontologists have believed for a century that organisms naturally evolve to larger and larger sizes. But in a new study, University paleontologist David Jablonski shows that this is not true. In fact, there is no more tendency for species to become bigger as they evolve than there is for them to become smaller. Jablonski's surprising findings are published in the Thursday, Jan. 16, issue of the journal Nature.

E.D. Cope's now-famous rule, that evolutionary lineages have a tendency to evolve toward larger body size, was first published in 1896 and is still widely cited in textbooks and professional scientific literature.

"Although Cope's Rule has been questioned for some time on a theoretical basis, this is the first time anyone has taken a quantitative look at a large enough data base to really draw a general conclusion," said Jablonski, Professor in Geophysical Sciences. "This is the empirical 'nail in the coffin.' "

Jablonski said that Cope, as have many others since him, tended to focus only on the largest animals in a given evolutionary lineage, and to target only those lineages that did increase in size. This is a sampling bias that skews the data.

"Dinosaurs are a great example," said Jablonski. "People forget that there were plenty of tiny dinosaurs running around, even at the end of the group's history. The evolution of the horse, from tiny Eohippus to the modern horse, is often cited as the classic example of Cope's Rule, but horses actually show a broad range of sizes through most of their evolutionary history -- until the very end, when all became extinct except for one of the largest lineages. The last survivor just happened to be a large one. If you connect the small starting point with the big final survivor, you seem to get a straight line of size increase, but the real pattern is much more complicated."

Jablonski said that paleontologists have to think about the whole range of body sizes, not just the extremes, when examining evolutionary trends.

Jablonski gave two reasons why Cope's Rule has so thoroughly permeated paleontological thinking, and both of them are psychological. "Believing that larger body size bestowed long-term evolutionary advantages fit our preconceptions that body size is important in the short term. Size determines who you can eat and who eats you, how widely you can range and your mating success. Secondly, there is a human tendency to focus on the largest animals: the biggest horse in a given time slice, for example.

"But my data show that although body size may be extremely important in an ecological sense, there is no simple extrapolation to size being important in a long-term, large-scale evolutionary sense. Size really matters ecologically, but it plays such a complex role on the larger evolutionary scale that there is no long-term, overarching pattern."

Being small has its evolutionary advantages. "You can survive when there are limited resources, you can reproduce rapidly, and you may be able to evade predators by being too small to catch -- rather than too big to take down," Jablonski said. "There is an infinity of possible ecological pressures on body size."

In examining Cope's Rule, Jablonski tracked the body-size range of 190 evolutionary lineages of mollusks over a broad geographic area and time span -- from New Jersey to Texas and through 16 million years near the end of the Cretaceous period. For 10 years he pored over museum collections and performed over 6,000 measurements of 1,000 different species.

Jablonski's data show no evolutionary preference for large body size over small, or small over large, for that matter. As many evolutionary lineages -- 27 percent -- had an overall body-size decrease as had an overall body-size increase. Twenty-eight percent indicated an increase at both ends of the size scale (both bigger and smaller). The smallest number of lineages, 9 percent, recorded a decrease in the total range of body sizes.

"This is exactly consistent with theoretical expectations -- if you believe that body size confers no singular advantage -- but very much in opposition to the classical Cope's Rule," Jablonski said.

Jablonski's research was funded by the National Science Foundation.

-- Diana Steele