GENETIC DIVERSITY AND TAXONOMIC PRIORITIES FOR CONSERVATION

Crozier (Biol. Conserv., 61, 1992) has proposed two measures for taxon -weighting, a uniqueness index and a product-formula, both based on ge netic distances or branch lengths interpreted as estimates of probabil ities of evolutionary change. I provide new versions of each measure t hat correct limitations in their original formulation. However, I also demonstrate other undesirable properties of these measures, including :failure to reflect complementarity among taxa, incompatibility with t he additivity property of branch lengths, and poor prediction of under lying feature diversity of taxon subsets. The examples presented here also counter Crozier's conjecture that the product-formula measure wil l tend to yield results similar to the phylogenetic diversity (PD) mea sure of Faith (Biol. Conserv., 61, 1992). An alternative probabilistic interpretation of branch lengths, based on a Poisson distribution, av oids the problems of Crozier's probabilistic measures. The correspondi ng probabilistic measure derived here is shown to be monotonically rel ated to PD, and so provides a further probabilistic rationale for phyl ogenetic diversity as an indicator of underlying feature diversity pat terns among taxa.