Showing papers by "John C. Avise published in 1974"

Systematic Value of Electrophoretic Data

Abstract: 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 of their 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 criteria. 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

Biochemical genetics of sunfish. I. Geographic variation and subspecific intergradation in the bluegill, Lepomis macrochirus

Abstract: Zones of secondary contact between allopatrically evolved populations have long been of particular interest to students of evolution. Former isolates often have not evolved fully efficient isolating mechanisms before rejoining and hybridization results. With the acquisition of electrophoretic techniques, the genetic aspects of natural hybridization can be examined in detail. Patterns of introgression of alleles are expected to differ depending largely on the extent to which the populations' gene pools have diverged and on the intensity of selection against \"the infiltration of genes from one balanced complex into another\" (Mayr, 1963). Theoretically, situations should be found representing a continuum from hybridization with no introgression, to free gene exchange resulting in the eventual fusion of gene pools. Genic and/or chromosomal imbalance may lead to hybrid sterility and restrict allelic exchange to the F 1 generation. Patton et al. (1972) found no evidence of genic introgression between two hybridizing species of gophers in Arizona. Lowered fitness of certain recombinant types may greatly limit allelic introgression. Hall and Selander (1973) found evidence for a very low level of introgression between karyotypically distinct \"F6\" and \"PI\" populations formerly placed in the lizard species Sceloporus grammicus. Selander et al. (1969) and Hunt and Selander (1973), working with semispecies of house mice, found steep gradients of transition in genic character across an area of contact, and differential extents of introgression of alleles among loci. They

Biochemical Polymorphism and Systematics in the Genus Peromyscus. VI. The Boylii Species Group

Abstract: An analysis of electrophoretic variation in proteins encoded by 21 genetic loci in 275 individuals belonging to the Peromyscus boylii species group yielded the following systematic conclusions. Populations of P. boylii rowleyi and P. b. levipes in four Mexican states and four states in the southwestern United States, separated by up to 3000 kilometers, share common alleles at virtually all loci, and thus show no evidence of representing more than one species. P. (b.) attwateri from Arkansas differs from P. boylii in allelic composition at several loci, and in all probability represents a distinct species, as suggested by other authors on the basis of morphology and karyotype. P. stephani from the Gulf of California is very similar genically to P. boylii, and, on the basis of this and other evidence, should be removed from the subgenus Haplomylomys and placed in the boylii species group of the subgenus Peromyscus. Populations referable to P. pectoralis from Ciudad Victoria, Mexico, Ranger, Texas, and Big Bend, Texas, show considerable allelic differences from one another but form a single cluster in a dendrogram of biochemical similarities. The nature of the allelic differences suggests that two or more species currently are classified as P. pectoralis.

Biochemical genetics of sunfish. ii. genic similarity between hybridizing species

Abstract: Recent improvements in electrophoretic and biochemical staining techniques have allowed a quantification of levels of genic similarity between various taxa and have stimulated renewed interest in the role of speciation in converting intraspecific to interspecific variation (Lewontin 1967). Mutationists in the early 1900s claimed that certain types of single mutations could give rise to new species. However, an examination of the tremendous amount of variability in interspecific F2 hybrids soon showed that even closely related species could differ at a large number of genetic loci (Harland 1936; Sumner 1932; Darlington 1940; Timofeeff-Ressovsky 1940). Electrophoretic studies have generally supported the latter observation. Populations belonging to closely related species usually show differences in allelic composition at a far higher percentage of genetic loci than conspecific populations. Studies on sibling species of Drosophila (Hubby and Throckmorton 1965; Nair et al. 1971; Ayala et al. 1970), Peromyscus (Smith, Selander, and Johnson, in preparation), kangaroo rats (Dipodomys) (Johnson and Selander 1971), and cotton rats (Sigmodon) (Johnson et al. 1972) indicate that even sibling species show major differences at between 20%o and 50% or more of their genetic loci and that "in spite of their morphological and ecological similarity and their evolutionary affinity these species have very different gene pools" (Ayala et al. 1970). These and similar studies provide a base with which genic similarities between other species may be compared. In this study, we examine protein differences in a group of fishes whose propensity to hybridize is well known. Natural interspecific hybridization is more common in fishes than in any other group of vertebrates (Lagler et al. 1962), and no group of fishes is more renowned for its ability to hybridize (with the possible exception of certain cyprinids) than are the centrarchids and particularly species of Lepomis (Birdsong and Yerger 1967). Theoretically the 11 species of Lepomis can produce 55 F1 interspecific hybrid types, and of these, at least 21 have been found in nature (Childers 1967). Artificially raised hybrids are frequently fertile (West 1970), and Hubbs (1955) has noted that introgressive hybridization may occasionally occur in nature.