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

Systematic Value of Electrophoretic Data

Species and Speciation in Phylogenetic Systematics, with Examples from the North American Fish Fauna

Phylogenetic relations among percid fishes as inferred from mitochondrial cytochrome b DNA sequence data.

Discussions aimed at resolution of the Tree of Life are most often focused on the interrelationships of major organismal lineages. In this study, we focus on the resolution of some of the most apical branches in the Tree of Life through exploration of the phylogenetic relationships of darters, a species-rich clade of North American freshwater fishes. With a near-complete taxon sampling of close to 250 species, we aim to investigate strategies for efficient multilocus data sampling and the estimation of divergence times using relaxed-clock methods when a clade lacks a fossil record. Our phylogenetic data set comprises a single mitochondrial DNA (mtDNA) gene and two nuclear genes sampled from 245 of the 248 darter species. This dense sampling allows us to determine if a modest amount of nuclear DNA sequence data can resolve relationships among closely related animal species. Darters lack a fossil record to provide age calibration priors in relaxed-clock analyses. Therefore, we use a near-complete species-sampled phylogeny of the perciform clade Centrarchidae, which has a rich fossil record, to assess two distinct strategies of external calibration in relaxed-clock divergence time estimates of darters: using ages inferred from the fossil record and molecular evolutionary rate estimates. Comparison of Bayesian phylogenies inferred from mtDNA and nuclear genes reveals that heterospecific mtDNA is present in approximately 12.5% of all darter species. We identify three patterns of mtDNA introgression in darters: proximal mtDNA transfer, which involves the transfer of mtDNA among extant and sympatric darter species, indeterminate introgression, which involves the transfer of mtDNA from a lineage that cannot be confidently identified because the introgressed haplotypes are not clearly referable to mtDNA haplotypes in any recognized species, and deep introgression, which is characterized by species diversification within a recipient clade subsequent to the transfer of heterospecific mtDNA. The results of our analyses indicate that DNA sequences sampled from single-copy nuclear genes can provide appreciable phylogenetic resolution for closely related animal species. A well-resolved near-complete species-sampled phylogeny of darters was estimated with Bayesian methods using a concatenated mtDNA and nuclear gene data set with all identified heterospecific mtDNA haplotypes treated as missing data. The relaxed-clock analyses resulted in very similar posterior age estimates across the three sampled genes and methods of calibration and therefore offer a viable strategy for estimating divergence times for clades that lack a fossil record. In addition, an informative rank-free clade-based classification of darters that preserves the rich history of nomenclature in the group and provides formal taxonomic communication of darter clades was constructed using the mtDNA and nuclear gene phylogeny. On the whole, the appeal of mtDNA for phylogeny inference among closely related animal species is diminished by the observations of extensive mtDNA introgression and by finding appreciable phylogenetic signal in a modest sampling of nuclear genes in our phylogenetic analyses of darters.

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Phylogeny and temporal diversification of darters (Percidae: Etheostomatinae).

The Ouachita Mountain Province in southwestern Oklahoma and southeastern Arkansas is an uplifted and strongly faulted region drained largely by the Kiamichi, Little, Ouachita and Saline rivers. Although a small region relative to other highland provinces in North America the fauna is distinctive, with at least eight endemic fish species. Congruent with these fish distributions are the distributions of five species of endemic crayfishes and four species of endemic salamanders. The distinctive aquatic fauna presumably evolved in a preglacial highland drainage system extending from the Blue River, Oklahoma to the Saline River in Arkansas. Biogeographically, several generalized tracks are discernable when species relationships are considered. The first track is one primarily inclusive of the Ozark Plateaus to the north. The second includes the Ouachita and Ozark highlands and highland regions east of the Mississippi valley and east to the Appalachian Mountains. Among fishes, two species groups and several allopatric populations of species exhibit this pattern. Congruent with this pattern are three species groups of crayfishes and one group of salamanders. Additional tracks are observed between rivers of the Ouachita Mountains and those surrounding the highlands or to the south in the Coastal Plain from Texas to the Atlantic Slope. Five species groups of fishes suggest this pattern and three species groups of crayfishes and one group of salamanders display congruence. Geological events hypothesized to be instrumental in the development of the first pattern include the development of the ancestral Arkansas River and glacial advance in the north. It is proposed that prior to the Pleistocene a highland topography and biota occurred not only in their present locations, but also in areas between the Ouachita and Ozark provinces and in the Central Lowlands Province north of existing highlands. The once continuous western highlands were vicariated with the development of the Arkansas River and its associated lowlands, prior to the Sangamonian interglacial. A northern vicariance occurred with the advance of glaciers and the deposition of their associated till in streams, creating habitats unsuitable for highland fishes. Ouachita endemics demonstrating the second generalized pattern might have either invaded the highland region or represent an old fauna present during the most recent uplift, in the early Tertiary.

Biogeography of ouachita highland fishes

1 1 Lactate dehydrogenase isozyme patterns were obtained for sixty-seven species of darters, tetrazolium oxidase electrophoretic mobilities for sixty-eight species and malate dehydrogenase isozyme patterns for fifty-seven species 2 2 The patterns and mobilities supported perviously assumed monophyletic origins for the genera Percina and Etheostoma 3 3 Several accepted relationships among darters were supported, and new relationship were suggested 4 4 Geographic variation was limited; the most significant variation was the dichotomous nature of the LDH pattern of P copelandi suggesting that the nominal species may actually consist of two or more taxonomically recognizable forms

L.M. Page

Papers

Lactate dehydrogenase isozymes, malate dehyrogenase isozymes and tetrazolium oxidase mobilities of darters (etheostomatini)