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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Evidence is presented that a geographically peripheral population of the annual Stephanomcria exigua ssp. coronaria (Compositae), a widespread and ecologically diverse species, has recently given rise by a process of sympatric speciation to a diploid species presently designated “Malheurensis.” The new species comprises less than 250 individuals and is found only at a single locality in eastern Oregon where it grows interspersed with its parental population. Stephanomeria exigua ssp. coronaria is an obligate outcrosser and “Malheurensis” is highly self‐pollinating. Reproductive isolation is maintained by differences in breeding system, a crossability barrier that reduces seed set following cross‐pollination between them, and reduction in hybrid fertility caused by chromosomal structural differences. They are very similar morphologically. Electrophoretic analyses of seven enzyme systems demonstrate that all the alleles but one at the controlling 13 gene loci in “Malheurensis” are identical to alleles in ssp. coronaria. The new species displays certain maladapted features including loss of the specific requirements for seed germination characteristic of the progenitor population of ssp. coronaria. The origin of “Malheurensis” appears to be an exception to the theory of geographical speciation because spatial isolation is not necessary at any time for the origin or establishment of its reproductive isolating barriers. The nature of these barriers plus the genetic homogeneity of the species are compatible with the hypothesis that it derives from a single progenitor individual. Very little genetic change is involved initially in this mode of speciation.

Differencesin the gametic chromosome numbers (n = 4, 5, 9) ofspecies.in the Astereae tribe ofthe Compositae have been variously .interpreted. One hypothesis proposes that n = 9 was the original base number ofthe group and that the lower numbers resulted from aneuploid reduction. The alternative hypothesis asserts that the ancestral base number was n = 4 or n = 5 and that species in which n = 9 are allotetraploids derived by hybridization between taxa with the lower numbers. Electrophoretic analysis of 17 enzyme systems' in.five species of Machaeranthera, in which n = 4, 5, and 9, and two species of Aster in which n = 5 and 9,-demonstrates that all of these species have the same number of gene loci specifying the tested enzymes. The absence of isozyme multiplicity in the species in which n = 9 suggests that they did not arise by polyploidy.Ploidy level in plants has usually been determined primarily on the basis ofchromosome number. Although satisfactory-in most cases, sharp differences characterize the interpretation ofchromosome numbers in the Astereae tribe of the Compositae. The most common chromosome number in the tribe is n = 9, but the numbers vary between n = 2 and n = 9, with many species having n = 4 or n = 5. One hypothesis proposes that n = 9 was the original base number of the group and that lower numbers resulted from aneuploid reduction (1). Evidence claimed to support this view is the association of n = 9 with the primitive woody habit, the high symmetry of the n = 9 karyotypes, and the widespread occurrence of this chromosome number in divergent lineages within the tribe. The alternative hypothesis calls attention to the rarity of species with the intermediate chromosome numbers n = 6 and n = 7 and asserts that the ancestral base number was n = 4.or n = 5, so that species in which n = 9 are allotetraploids derived by hybridization 'between taxa with the lower numbers (2, 3).The essential attribute ofpolyploidy, however, is not relative chromosome number but genome multiplication and its attendant increases in number ofgene loci. Consequently, I have used gel electrophoresis to determine whether species in which n = 9 have more gene loci coding particular isozymes than those in which n = 4 or 5. The approach builds on two recent findings: (i) allopolyploid species display isozyme multiplicity relative to diploids because they inherit from their diploid parents homoeologous loci that are frequently fixed for different alleles (4-6) and (ii) decrease in chromosome number from the ancestral diploid level as a result ofaneuploidy does not change the number of structural genes specifying isozymes (7). Thus, .if species in which n = 9 have many more isozyme loci than those in which n = 4 or n = 5, they are likely to be polyploid. Polyploidy is rejected if they have the same number ofloci. (Jackson 7640, Beatty NV). The seeds were generously provided by R. C. Jackson. Several-ofthe collections had been previously grown for electrophoretic~studies (8). MATERIALS AND METHODSThe seeds were germinat...

SummaryThis paper reviews the evidence that three pairs of diploid annual plant species are related as progenitor and recent derivative. The species pairs are Layia glandulosa and Layia discoidea , Clarkia biloba and Clarkia lingulata , and Stephanomeria exigua ssp. coronaria and Stephanomeria malheurensis . The three cases are examples of Verne Grant's model of 'Quantum Speciation', in which a derived species is budded off and acquires new traits while the parental species continues more or less as before. The derived species differ from their progenitors in different ways and show different modes of reproductive isolation. However, the number of differences between each derivative and its progenitor appears to be few and relatively simple in genetic terms. Comparison of a recently evolved species with its progenitor can reveal what happens during speciation because overall divergence is minimal and the direction of evolution is clear. Evidence from DNA sequences may be particularly useful to recognize additional examples of progenitor and derivative relationship.