Electrophoretic Evidence and Plant Systematics

Abstract

The study of phenotypes and their variation often provides evidence for phylogenetic inferences in plant systematics. Therefore, it is critical that the phenotypes analyzed reflect as directly as possible the underlying genotypes. The equation between phenotype and genotype is simpler and better understood for evidence obtained by electrophoresis of plant enzymes than for most morphological characters. This article discusses the advantages and limitations of electrophoretic evidence to test hypotheses in plant systematics and evolution. It also summarizes the results of a large number of studies which have utilized this evidence. Three general observations from these studies are: (1). Conspecific plant populations are extremely similar genetically as documented by their very high mean genetic identities, 0.95 + 0.02. This result suggests that one or a few populations often constitute an adequate sample of a species. (2). Congeneric plant species have strikingly reduced mean genetic identities, 0.67 + 0.07. However, certain pairs of annual plant species have genetic identities similar to those of conspecific populations. In these cases, the species have been shown to be related as progenitor and derivative with the derivative being of recent origin. (3). The amount of genetic variability within plant populations appears closely correlated with their breeding system, with outcrossing populations substantially more variable than inbreeding ones. The article also describes a number of actual and potential applications of electrophoresis in plant systematics. Evidence obtained by electrophoresis of enzymes has not been widely utilized by plant systematists although it has dominated the research of many of their zoological counterparts and population geneticists (Manwell & Baker, 1970; Lewontin, 1974; Nei, 1975; Ayala, 1976). This has meant that the strengths and weaknesses of such evidence for solving systematic and evolutionary questions in plant biology have not been sufficiently discussed. The present article is designed to facilitate an efficient evaluation, and emphasizes the unique characteristics of electrophoretic evidence, the requirements for its analysis, and actual and potential applications in plant systematics and evolution.