Which came first--trait or environment? This question lies at the heart of the study of adaptation by natural selection. In Mediterranean-type climates, many woody plants have small, tough, evergreen leaves (dubbed 'sclerophylls') which allow them to photosynthesize during the cool wet winters and survive the hot summer dry season. This paper presents the first comparative test of alternative hypotheses to explain these shared features. Did these traits evolve independently in different lineages, in response to the unique Mediterranean-type climate? Alternatively, did the traits evolve first in response to other factors, subsequently promoting the success of these lineages when summer drought conditions arose in the past few million years? Twelve independent plant lineages of the California chaparral were examined, using comparative methods that combine new phylogenetic information and statistical techniques. Only four of the twelve lineages showed the pattern expected if the leaves had evolved to suit the Mediterranean-type climate. In the other cases, the leaf traits appear to be ancestral characteristics of these lineages that predate the climatic conditions. These results, together with biogeographic and paleobotanical information, highlight the importance of ecological processes in concert with adaptive evolution in shaping the form and function of organisms in modern communities.
For more information about this article and other articles in the May issue, see The American Naturalist online: www.journals.uchicago.edu/AN