GENETIC DIVERSITY AND THE AGONY OF CHOICE

Vane-Wright et al. (Biol. Conserv., 55, 1991) correctly stress the importance of using the evolutionary distinctiveness of taxa when assigning them priorities for preservation-'the agony of choice'. In the absence of other information, the crucial quantity is the preservation of genetic diversity. The method of Vane-Wright et al. uses exclusively the topology of the inferred phylogeny of the group. This cladistic method will err on occasion because it does not take into account the accumulation of genetic divergence along branches of the evolutionary tree, and because it relies on the tree being rooted. When available, the use of reliable genetic data following tree construction is preferable. The species with the highest priority for conservation is then that with the highest overall probability of having unique character states. If there is a linear relationship between distance and the probability of character state change, the relative probability of uniqueness of a taxon may be estimated by the product of its genetic distances to other taxa along the branches of the dendrogram. Habitats or reserve systems can be ranked according either to the priorities assigned to their constituent species. or their preservation of biodiversity estimated by using a simple product rule applied to the lengths of the branches conserved.