A review of the use of allozyme electrophoresis in plant systematics.

The role of electrophoretic data is discussed as it applies to plant taxonomy and systematic studies. Nei's (Am. Nat. 106 (1972) 283-292; Genetics 89 (1978) 583-590) genetic distances calculated for a large number of populations, species and genera were taken from published data. The relation between Nei's genetic identity measures and taxonomic rank (populations, species and genera) are shown graphically. The graphs obtained in this way (from 3021 pairs of plant taxa) differ substantially from previous graphs published by Thorpe (Ann. Rev. Ecol. Syst. 13 (1982) 139-168; in: G.S. Oxford, D. Rollinson (Eds.), Protein Polymorphism: Adaptive and Taxonomic Significance, Academic Press, London, 1983, pp. 131-152) and Thorpe and Solé-Cava (Zool. Scripta 23 (1994) 3-18). These authors suggested that the divergence between the different taxonomic ranks is roughly similar across a wide range of taxa. The latter was based on values for 2664 (Thorpe, 1982) and 8060 (Thorpe, 1983) pairs of animal and plant taxa, but the plant data contributed little to the total. For any given taxonomic rank, we found that plants are genetically more closely related than animals (possibly with the exception of birds). This result is important because the empirical relationships of genetic distance measures, to different levels of taxonomic separation, is often used for distinguishing and identifying cryptic or sibling species where conventional methods are unable to resolve systematic problems.