The study of plienotypcs and their \ ariation often provides evidence for phylogenctic inferences in plant systeniatics. 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 morpliological characters. This article discusses the advantages and limitations of electrophoretic evidence to test hypotheses in plant systeniatics 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 c(mstitute an adequate sample of a species. (2). Congeneric plant species ha\e strikingly reduced mean genetic identities, O.fiT ± 0.07. However, certain pairs of annual plant species have genetic identities similar to those of conspecific populations. In these cases, die species ha\c been shown to be related as progenitor and derivative with the derivative being of recent origin. (3). The auKumt of genetic variability within plant populations appears closely correlated with their breeding system, witli outcrossing populations substantially more variable than inbreeding ones. The article also describes a numl)er of actual and potential applications of electrophoresis in i^lant systeniatics.Evidence obtained by electrophoresis of enzymes has not been widely utilized by plant systeniatists 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. Electiwpiiohktic Evidence: Advantages and LimitationsThe systematist analyzes phenotypes and their variation and often uses this evidence for phylogenctic inference. Such inferences recjuire that observed phenotypes have a specifiable relationship to unobserved genotypes. The equation between phenotype and genotype is simpler and better understood for electrophoretic evidence than it is for evidence obtained from morphological characters or chromatographic comparisons of secondary metabolites. This follows from the colinearity of amino acid sequence and nucleotide sequence as well as the specificity of enzyme catalysis. It also reflects the fact that electrophoretic evidence is used to answer a very different kind of question than has usually been posed by systematists.Morpho...

Many enzymes in plants have isozymes because the same catalytic reaction is often present in several subcellular compartments, most frequently the plastids and the cytosol. The number and subcellular locations of the isozymes appear to be highly conserved in plant evolution. However, gene duplication in diploid species and the addition of genomes in polyploid species have increased the number of isozymes.

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