But a study published this week by two biologists at the University of California, San Diego in an early online edition of the Proceedings of the National Academy of Sciences suggests that maxim, known as "Cope's Rule," may be only partly true.
The scientists found that populations of tiny crustaceans retrieved from deep-sea sediments over the past 40 million years grew bigger and evolved into larger species, as might be predicted from Cope's Rule. However, the changes in the sizes of these clam-like crustaceans commonly known as ostracodes --from the genus Poseidonamicus -- increased only when the global ocean temperature cooled. When temperatures remained stable, not much happened to body size.
"These data show a very nice correlation between temperature and body size," said Kaustuv Roy, a professor of biology at UCSD and a coauthor of the paper.
"Although not the most glamorous of fossils, deep-sea ostracodes are very useful for this question because they have a rich fossil record, which allows us to reconstruct the evolution of body size in great detail," said Gene Hunt, who designed and conducted the study while postdoctoral fellow at UCSD.
"Scientists have been interested in how body size evolves for a long time, but there is a lot of uncertainty about what factors are most important in determining whether animals get bigger or smaller over time," added Hunt, now a curator at the National Museum of Natural History in Washington, DC.
The two scientists said their data suggest that Cope's Rule--named for Edward Cope, a 19 th century American paleontologist who claimed the fossil record showed that lineages became larger over time--may simply be an evolutionary manifestation of Bergmann's Rule, which holds that animals increase in m
Source:University of California - San Diego