Systematic Value of Electrophoretic Data

Avise, John C. (Department of Genetics, University of California, Davis, California 95616) 1975. Systematic value of electrophoretic data. Syst. Zool. 23:465-481.-Two consistent observations from recent multi-locus electrophoretic studies are: (1) levels of genic similarity between conspecific populations appear very high (populations nearly identical in allelic content at 85 percent or more of their loci) and (2) genic similarities between different, even very closely related species, are generally much lower and more widely dispersed (congeneric species pairs often completely distinct at one-fifth to four-fifths oftheir loci). These observations have valuable implications regarding the practical utility of electrophoresis: (1) one or a few samples often yield adequate data for the description of an entire species for systematic purposes and (2) closely related species may be arranged according to percentages of shared alleles or genotypes. A survey of the literature indicates that when such arrangements are made, they usually correspond very closely to previously recognized relationships of various species groups based on classical systematic riteria. This observation, coupled with several theoretical advantages of the study of allozymes, makes it clear that electrophoretic techniques will provide an extremely valuable tool for systematists. [Electrophoresis; genetics; systematics.] Electrophoretic techniques were first used by Tiselius (1937; cited by Brewer, 1970) to distinguish multiple fractions of serum proteins migrating through solution under the influence of an electric current. During the next 25 years, advances in electrophoretic methodology and knowledge centered on three fronts: (1) improvements in types of supporting media including the development of starch gels (Smithies, 1955) which are widely used today; (2) the application of histochemical staining methods (Hunter and Markert, 1957), which allowed analysis of electrophoretic variation in enzymatic proteins; and (3) the demonstration that much of the variation was inherited in simple Mendelian fashion. Prior to 1963, most studies described variation in single proteins, but by the mid 1960's, electrophoretic techniques were sufficiently refined to permit examinations of large numbers of different proteins in the same organisms (Hubby, 1963; Hubby and Throckmorton, 1965; Hubby and Lewontin, 1966; Johnson et al., 1966; Lewontin and Hubby, 1966; Harris, 1966). These multi-loci studies were the prototypes for a profitable new method of analysis of levels of genic variability and population structure (review by Gottlieb, 1971). Ironically, some of their findings may also have had an initially retarding influence on the evaluation of electrophoretic data in systematics because, although it was immediately recognized that a quantification of allozyme differences between populations (based on allele or genotype frequencies) might offer potentially valuable information for systematics (Hubby and Throckmorton, 1965), other allozymic results stimulated far greater interest. The disclosure of very high levels of genic variability in natural populations was in apparent conflict with classical population genetic models of balanced load, and hence generated great controversy. Researchers turned their attention to the question of whether most allozymes were maintained by natural selection or were selectively neutral (Proc. VI Berkeley Symp., 1972). Thus, even by 1970, a major review of electrophoretic literature included almost no discussion of uses of electrophoretic data in systematics, other than for description and identification of species (Manwell and Baker, 1970). Today, the selectionist versus