Things don't always become bigger - Univ of Chicago paleontologist David Jablonski disputes Cope's Rule that organisms naturally evolve to larger sizes - Brief Article

USA Today (Society for the Advancement of Education), June, 1997

Paleontologists have believed for a century that organisms naturally evolved to larger and larger sizes. However, a study by University of Chicago paleontologist David Jablonski shows that this is not true. In fact, there is no more tendency for things to become bigger as they evolve than there is for them to become smaller.

Cope's Rule that evolutionary lineages have a tendency to evolve toward larger body size is one of the most long-standing evolutionary "laws." E.D. Cope first formulated his now-famous rule in a book published in 1896, and it is cited widely in textbooks from elementary to graduate school.

"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," Jablonski indicates. "This is the empirical `nail in the coffin.'" Jablonski maintains that Cope, as have many others since him, tended to focus only on the largest animals in a given evolutionary lineage and to home in on only those lineages that did increase in size. This is a sampling bias that skews the data.

"Dinosaurs are a great example. People forget that there were plenty of tiny dinosaurs running around, even at the end of the group's history. And the evolution of the horse, from tiny Eohippus to the modern horse, is often cited as the classic example of Cope's Rule. But, in fact, horses 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 argues that paleontologists have to think about the entire range of body sizes when examining evolutionary trends, not just the extremes. He says that there are two reasons why Cope's Rule has permeated paleontological thinking so thoroughly, and both of them are psychological. "Size determines who you can eat and who eats you, how widely you can range and your mating success. Believing that larger body size bestowed long-term evolutionary advantages fit our preconceptions that body size is important in the short term. 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 tremendously 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."

Besides, there are evolutionary advantages to being small. "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. 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 large geographic area and time span -- from New Jersey to Texas and through 16,000,000 years -- near the end of the Cretaceous period. His data show that there is no evolutionary preference for large body size over small, or small over large, for that matter. As many evolutionary lineages -- 27% -- demonstrate an over-all body size decrease as an over-all increase. Twenty-eight percent show an increase at both ends of the size scale (both bigger and smaller). The smallest number of lineages -- nine percent -- had 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."