Convergent evolution, the independent occurrence of phenotypic similarity, is a widespread and common phenomenon. Methods have been developed to identify instances of convergence, but there is a lack of techniques for quantifying the strength of convergence. We therefore investigated whether convergent evolution can be quantified in a meaningful way.
We have developed a simple metric (the Wheatsheaf index) that provides an index of the strength of convergent evolution incorporating both phenotypic similarity and phylogenetic relatedness. The index is comparable across any quantitative or semiquantitative traits and thus will enable the testing of various hypotheses relating to convergence.
The index performs well over a range of conditions. We apply it to an empirical example using Anolis lizard ecomorphs to demonstrate how it can be used.
The Wheatsheaf index provides an additional tool that complements methods aimed at identifying cases of convergent evolution. It will enable cases of convergence to be analysed in more detail, test hypotheses about its mechanics as an evolutionary process and, more generally, the predictability of evolution (how often do we see strong convergence and does this mean evolutionary solutions are limited?).