Stephen G. Tilley

The evolution of asymmetry in sexual isolation : a model and a test case

We constructed a model for the evolution of sexual isolation by extending Landes (1981) model of sexual selection. The model predicts that asymmetric sexual isolation is a transient phenomenon, characteristic of intermediate stages of divergence in sexually selected traits. Unlike the Kaneshiro (1976, 1980) proposal, our model does not depend upon drift and the loss of courtship elements to produce asymmetries in sexual isolation. According to our model, the direction of evolution cannot be predicted from asymmetry in sexual isolation. We tested some features of the model using data from an experimental study of sexual isolation in the salamander Desmognathus ochrophaeus. We tested for sexual isolation between 12 allopatric populations and found significant asymmetry in sexual isolation in about a quarter of the test cases. The highest degrees of asymmetry were associated with intermediate levels of divergence. A curvilinear relationship between isolation asymmetry and divergence was predicted by our model and was supported by statistical analysis of the salamander data.

Life Histories and Comparative Demography of Two Salamander Populations

Two populations of the plethodontid salamander Desmognathus ochrophaeus inhabiting wet, vertical rockfaces in the southern Appalachian Mtns. were censused by the Jolly-Seber multiple recapture method over a 7-yr period. The populations were known to differ in age at sexual maturity, and a primary goal of the study was to obtain survivorship estimates that would permit comparisons of overall life histories of the two populations. Low elevation juveniles grow faster than high elevation juveniles during fall, winter and spring of their first full year of life. They thereafter grow more slowly than equal-aged high elevation animals and become externally sexable in their third summer, a year earlier than juveniles of the high elevation population. During the summer high elevation individuals of all sizes grow faster than those of corresponding size and sex in the low elevation population, since growth rates decline with attainment of sexual maturity. Estimated densities were about 3 sexable males and 3 sexable females per m2 for both populations throughout the study. The co-occurrence of a potential predator and competitor, D. monticola, at the low elevation site had no discernible effect on density of D. ochrophaeus. Survivorship of sexable animals appears to be higher over the colder months at the high elevation site; survivorship from June to August appears similar for the two populations. Annual survival rates were estimated to be 0.743 and 0.626 for high and low elevation animals that are externally sexable. Recruitment appears to balance losses in both populations. Estimated probabilities of survival from hatching to first oviposition were 0.059 and 0.087 at the high and low elevation sites. The difference reflects differences in age at maturity. Fertility rates appear to be uncorrelated with age in both populations, but larger females lay more eggs so that clutch sizes are higher in the high elevation population.

GENETIC STRUCTURE OF THE BLUE RIDGE DUSKY SALAMANDER (DESMOGNATHUS ORESTES): INFERENCES FROM ALLOZYMES, MITOCHONDRIAL DNA, AND BEHAVIOR

Abstract The plethodontid salamander Desmognathus orestes, a member of the D. ochrophaeus species complex, is distributed in southwestern Virginia, eastern Tennessee, and western North Carolina. Previous allozyme analyses indicate that D. orestes consists of two distinct groups of populations (D. orestes‘B’ and D. orestes‘C’) with extensive intergradation and probable gene flow between these two groups. Spatially varying allele frequencies can reflect historical associations, current gene flow, or a combination of population‐level processes. To differentiate among these processes, we use multiple markers to further characterize divergence among populations of D. orestes and assess the degree of intergradation between D. orestes‘B’ and D. orestes‘C’, specifically investigating variation in allozymes, mitochondrial DNA (mtDNA), and reproductive behavior among populations. On a broad scale, the mtDNA genealogies reconstruct haplotype clades that correspond to the species identified from previous allozyme analyses. However, at a finer geographic scale, the distributions of the allozyme and mtDNA markers for D. orestes‘B’ and D. orestes‘C’ are discordant. MtDNA haplotypes corresponding to D. orestes‘B’ are more broadly distributed across western North Carolina than predicted by allozyme data, and the region of intergradation with D. orestes‘C’ indicates asymmetric gene flow of these markers. Asymmetric mating may contribute to observed discordance in nuclear versus cytoplasmic markers. Results support describing D. orestes as a single species and emphasize the importance of using multiple markers to examine fine‐scale patterns and elucidate evolutionary processes affecting gene flow when making species‐level taxonomic decisions.

Studies of Life Histories and Reproduction in North American Plethodontid Salamanders

Plethodontid salamanders are among the most abundant vertebrates of many North American ecological communities. Their evident ecological importance (Burton and Likens 1975a, 1975b) and unique associations of ubiquity, obscurity, and beauty have popularized plethodontids as subjects of evolutionary and ecological studies. Their reproductive ecologies and life histories have received particular attention in recent years, as studies have progressed from scattered anecdotes to detailed investigations. Sal-the and Mecham (1974) reviewed the literature through the 1960s. Since then, studies of several species have appeared: Gyrinophilus porphyriticus (Bruce 1969, 1972), Pseudotriton montanus (Bruce 1969, 1974, 1975), Stereochilus marginatus (Bruce 1971), Eurycea quadridigitata (Harrison 1973), Eurycea neotenes (Bruce 1976), Eurycea nana (Tupa and Davis 1976), Desmognathus fuscus (Danstedt 1975), Desmognathus ochrophaeus (Fitzpatrick 1973; Forester 1977; Huheey and Brandon 1973; Tilley 1972, 1973a, 1973b, 1974; Tilley and Hausman 1976), and Plethodon wehrlei (Hall and Stafford 1972). Houck reviews certain aspects of life histories of terrestrial plethodontids in this volume. Much, however, remains to be learned.

Allozymic Variation and Occurrence of Multiple Inseminations in Populations of the Salamander Desmognathus ochrophaeus

BOROWSKY, R. L. 1969. Ecological and social factors influencing the polymorphic pigmentation systems of Xiphophorus v. variatus (Pisces: Poeciliidae). Unpubl. Ph.D. Thesis, Yale University. , AND K. D. KALLMAN. 1976. Patterns of mating in natural populations of Xiphophorus. I: X. maculatus from Belize and Mexico. Evolution. In Press. DARNELL, R. M. 1962. Fishes of the Rio Tamesi and related coastal lagoons in East-Central Mexico. Publ. Inst. Mar. Sci. Port Aransas, Texas. 8:299-365. FARR, J. A. 1975. The role of predation in the evolution of social behavior of natural populations of the guppy, Poecilia reticulata (Pisces: Poeciliidae). Evolution 29:151-158. KALLMAN, K. D. 1970. Sex determination and the restriction of sex-linked pigment patterns to the X and Y chromosomes in populations of a poeciliid fish, Xiphophorus maculatus, from the Belize and Sibun rivers of British Honduras. Zoologica 55:1-16. . 1973. The sex-determining mechanism of the platyfish, Xiphophorus maculatus. p. 19-28. In: J. H. Schroder, ed. Genetics and mutagenesis of fish. Springer. , AND J. W. ATZ. 1966. Gene and chromosome homology in fishes of the genus Xiphophorus. Zoologica 5:107-135. , AND R. BOROWSKY. 1972. The genetics of gonopodial polymorphism in two species of poeciliid fish. Heredity 28:297-310. ROSEN, D. E. 1960. Middle American poeciliid fishes of the genus Xiphophorus. Bull. Florida State Mus., Biol. Sci. 5:57-242. WILSON, E. 0. 1975. Sociobiology: The new synthesis. Belknap Press of Harvard University Press. Cambridge. ZANDER, C. D. 1962. Untersuchungen iiber einen arttrennenden Mechanismus bei lebengebarenden Zahnkarpfen aus der Tribus Xiphophorini. Mitt. Hamburg. Zool. Mus. Inst. 60:205-264.

A NEW SPECIES OF BLACK-BELLIED SALAMANDER (GENUS DESMOGNATHUS) FROM THE APPALACHIAN MOUNTAINS OF NORTHERN GEORGIA

We describe a new species of Desmognathus from Union County, Georgia. It closely resembles D. quadramaculatus, but the two are distinguishable by adult size, body proportions, color pattern, and fixed differences at four allozyme loci. The new, smaller species is currently known from two tributaries of the Nottely River and is sympatric with D. quadramaculatus at both sites. The new form is semi-aquatic and utilizes habitats that are similar to those of D. quadramaculatus. Metamorphosis occurs at a smaller size than in D. quadramaculatus, apparently as a result of a shorter larval period. Selection on life history features may have had a role in the origin and divergence of these forms.

Electrophoretic Variation in Appalachian Populations of the Desmognathusfuscus Complex (Amphibia: Plethodontidae)

While it is distributed over much of eastern North America, Desmognathus fuscus is largely absent from the high mountains of the southern Blue Ridge Physiographic Province. Populations of salamanders allied to D. fuscus do occur at high elevations in the Great Smoky, Unicoi and Great Balsam mountains, and electrophoretic studies indicate that they represent a distinct, undescribed species. Both the undescribed form and D. fuscus occur in the Unicoi Mountains, where they are separated altitudinally, and where no gene exchange occurs between populations separated by only 6 km. The undescribed form is genetically more similar to southern than to northern populations of D. fuscus.

A Reinterpretation of the Reproductive Cycle and Demography of the Salamander Desmognathus ochrophaeus

and Charles W. Myers for their suggestions and assistance in the field. I especially thank the latter for the photographs for Figs. 9 and 11. Field and laboratory studies were made possible by support from the National Science Foundation (Grants GB-1441 and GB-5818), and field studies in Panama were carried out under the auspices of a grant from the National Institutes of Health (NIH GM-12020) in co-operation with the Gorgas Memorial Laboratory in Panama.

Sympatric Occurrence of Two Species of the Two-Lined Salamander (Eurycea bislineata) Complex

Abstract Genetic analyses of contact zones between closely related taxa are critical to an understanding of reproductive isolation between species. We evaluated allelic frequencies and external morphology from one such contact zone between two members of the Eurycea bislineata complex (i.e., E. cirrigera and E. wilderae). We found that, within this zone of contact, these presumed species had significantly different frequencies of alleles at three loci. In addition, these sympatric forms were significantly different in lateral mottling pattern, tail color, and length of tail stripe. These morphological patterns were identical to those used to describe the original subspecies E. b. cirrigera and E. b. wilderae. Evidence from this zone of contact supports the hypothesis that these forms are separate species. Moreover, there is evidence of ecological and/or reproductive character displacement among these species when in sympatry.

Ethological Isolation and Variation in Allozymes and Dorsolateral Pattern between Parapatric Forms in the Desmognathus ochrophaeus Complex

Questions regarding the nature of species (here defined as genetically cohesive, reproductively isolated groups) are central to understanding the evolutionary history of a group of organisms. Insight into the processes responsible for creating and maintaining genetically cohesive units can be attained by examining areas where such units contact one another. Clearly, it seems most pertinent to examine the maintenance of a species upon secondary contact with related taxa, assessing the genetic structure and possible outcomes of these interactions. Research on secondary contact has focused on hybrid zones, usually those marked by sharp clines in particular characters. A hybrid zone is characterized as an area where genetically distinct groups meet, mate, and produce some offspring of mixed ancestry (Barton and Hewitt, 1985; Harrison, 1990,Harrison, 1993). Upon secondary contact, a number of outcomes are possible. Narrow zones may be maintained by a balance between dispersal and selection against hybrids (Barton and Hewitt, 1985). Alternatively, there can be fusion of the two taxa (Hewitt, 1988) or replacement of one taxon by another (Shapiro, 1998). Various degrees of introgression may occur (Arnold and Bennett, 1993) or a hybrid swarm may develop (Heiser, 1947).