Comparing Plants and Connecting Traits

Jonathan Silvertown, Mike Dodd

Abstract

The diversity of plant life histories provides a wealth of raw material for comparative studies on evolution and ecology. The two fundamental questions for any comparative study are: which traits are correlated with one another, and are these correlations the result of common descent or convergent evolution? Phylogeny should therefore be explicitly included in any comparative analysis that is concerned with the causes of correlation between traits, even when the principal research question is a purely ecological one. In illustration, the method of phylogenetically independent contrasts (PIC) is used to test two long-standing hypotheses that have not been satisfactorily tested before. In the first example we find that annuals and species of early succession have greater reproductive allocation than perennials and species of later succession. In the second example we show that the apparency hypothesis of chemical defence is supported by a positive correlation between woodiness and the frequency of tannins and by a negative correlation between tannin frequency and alkaloid frequency. Finally, we point out that PIC has a much lower type-I error than cross-species analyses and that this superiority is surprisingly robust to lack of phylogenetic resolution.