Recently paleontologists decided to test the apparent trend that the body size of animals within a genus will become larger over time. Indeed, this trend, known as Cope's Rule, is exhibited by dinosaurs and mammals. In studies of fossil mollusks, however, Cope's Rule does not hold up. Researchers examined almost 200 genera of 75 million-year-old clams, oysters, and snails that had evolved for 16 million years. Mixed results were obtained. One-third of the genera did evolve bigger sizes, while one-third got smaller, and the remaining third contained some species that got smaller and some species that got larger. Theories explaining both why its advantageous to be big and why its advantageous to be small have been proposed. However, after this study on mollusks, scientists caution against claiming trends exist without actually proving them.
"Bigness Not Always a Plus, Mollusks Say," Science, January 17, 1997