Testing for evolutionary trade-offs in a phylogenetic context: ecological diversification and evolution of locomotor performance in emydid turtles


Patrick R. Stephens, National Center for Ecological Analysis and Synthesis, 735 State St., Suite 300, Santa Barbara, CA 93101, USA.
Tel.: +1 (805) 882 9218; fax: +1 (805) 892 2510; e-mail: stephens@nceas.ucsb.edu


The evolution of ecological trade-offs is an important component of ecological specialization and adaptive radiation. However, the pattern that would show that evolutionary trade-offs have occurred between traits among species has not been clearly defined. In this paper, we propose a phylogeny-based definition of an evolutionary trade-off, and apply it to an analysis of the evolution of trade-offs in locomotor performance in emydid turtles. We quantified aquatic and terrestrial speed and endurance for up to 16 species, including aquatic, semi-terrestrial and terrestrial emydids. Emydid phylogeny was reconstructed from morphological characters and nuclear and mitochondrial DNA sequences. Surprisingly, we find that there have been no trade-offs in aquatic and terrestrial speed among species. Instead, specialization to aquatic and terrestrial habitats seems to have involved trade-offs in speed and endurance. Given that trade-offs between speed and endurance may be widespread, they may underlie specialization to different habitats in many other groups.