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The Dynamics of Two Hybrid Zones in Appalachian Salamanders of the Genus Plethodon

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The Dynamics of Two Hybrid Zones in Appalachian Salamanders of the Genus Plethodon Evdzuion. 46(4). 1992, pp. 930-938 THE DYNAMICS OF TWO HYBRID ZONES IN APPALACHIAN SALAMANDERS OF THE GENUS PLETHODON / NELSON G. HNKXON, SR, R. HAVEN WILEY, AND CIEARLES K SMRI’ Department of Biology. University of North Carolina, Chapel Hill, NC 27599 US AND J / KENNERiAK.NEIDEL ,?’ Charlotte Latin School, 9502 Providence Road, Charlotte, NC 28226 UkA t Abstract.-Two zones of i&gradation between populations of PIethocfon have been studied for 18 and 20 years, respectively. The data consist of systematic scores of colors, made at least twice annually. Near Heintooga Overlook in the Balsam Mountains (Great Smoky Mountains National Park), the salamanders’ cheeks are gray. Proceeding north &ward the Smokies, there is increasing iizquency and intensity of red color at two, four, and six miles. There has been no change in the scores at any location. The width of the zone and our Eaiiure to detect any change can be explained by assuming neutrality of the character and random diffusion during the probable time since contact between the two in&grading forms, which most likely took place after the Hypsithermal Interval, 8,000-5,000 BP. At Coweeta Hydrologic Laboratory in the Nantahala Mountains, Plethodon jor- dani and P. glutinosus hybridize at intermediate elevations. The lateral white spots of gZutinosus demease and the red on the legs of jordani increases witl+vation &om 685 m to 1,052 m. At the higher elevation, the proportion of animals scored as ~fiure’* joraizni declined si&kantly from 1974 to 1990, an indication that the hybrid zone is qn+eading upward. The rate of spread is too great to be explained by random diffusion, so selection for gZutinosus charac&s is the best expla- nation. The rate of spread of the hybrid zone indicates that hybridization began 60-65 years ago, * at the end of the time of intense timbering. Such human dishubances have caused hybridization in other organ+Ils. Key words.-Disturbance, hybrid zone, North Carolina, paleoecology, spreading rate. Received February 4, 199 1. Accepted November 22, 199 1. Beginning with the period of the evolu- 1981a, 1981b;.Harrison and Arnold, 1982; tionary synthesis (Mayr, 1942,1963), zones Harrison, 1985, 1986; Arntzen and Wallis, of intergradation have been regarded as im- 199 1; Szymura and Barton, 199 l), as well - portant in understanding speciation and as in theory (Bazykin, 1969; Crosby, 1970; other aspects of the evolutionary process. It May et al., 1975; Endler, 1977; Barton, was recognized that steep clines could not 1979a, 19796, 1983; Barton and Charles- be maintained without either selection worth, 1984; Slatkin, 1973, 1975, 198 1, against-the hybrids or some other selective 1985). This recent literature, reviewed by force, such as that involving ecotones be- Barton and Hewitt (1985), and more exten- tween ecologically di&ent habitats to which sively by Harrison (1990), has stressed the the two intergrading populations were sep- conditions necessary for the stability of hy- arately adapted (Mayr, 1963). Without these brid zones, especially the importance of se- conditions, normal gene flow would spread lection against hybrids. The evidence has the different genes of the two populations included the absence or ratity of F2 and until the cline becomes nearly flat, its slope backcross individuals (Arntzen andWallis, depending on the rate of gene flow and the 199 l), or apparent linkage disequilibrium number of generations since the process be- in the absence of chromosomal linkage, in- gan. volving excess representation of parental In recent years, there have been signifi- ‘genomes in the hybrid zone (Szymura and cant advances in empirical studies (Little- Barton, 199 1). john, 1965; Littlejohn et al., 1971; Little- As noted by Harrison (1990), relatively john and Watson, 1985; Barton and Hewitt, few hybrid zones have been observed for more than brief periods. Most studies thus ’ Present address: Biology Department HP-2, High lack the time dimension, and interpretation Point College, High Point, NC 27261 USA. has involved an assumption of stability. 930 931 Nearly all of the studies that have spanned long periods have involved early and late observations, with few or none between (e.g., Rising, 1983; Moore and Buchanan, 1985; Hillis and Simmons, 1986; Turner, 1971; Amtzen and Wallis, 199 1). Szymura and Barton (199 1) provide an exception in the case of Bombina in eastern Europe, with data extending over a period of nearly 60 Y-- We report observations made twice an- nually for 18 to 20 years on two zones of intergradation between large Plethodon sal- FICA 1. Map of western North Carolina, showing amanders in the southern Appalachians (Fig. the locations of the two hybrid zones. Solid lines are 1). One of the zones is between two gee- riverx broken lines are state boundaries. graphic representatives of Pkthodon jor- dani that adjoin one another near the junc- ulation stability, population structure, in- tion of the Great Smoky Mountains and the terspecifrc competition, microdistribution Balsam Mountains of North Carolina. In and individual movements (Hairston, 1973, the Great Smoky Mountains, a range 77 km 1980a, 1980b, 1983a, 1983b; Hairston et long, the P. jordani population consists al- al., 1987; Kneidel, pers. obs.; Nishikawa, most entirely of red-checked individuals. 1985, 1990). For those u&miliar with Where the Smokies and Balsams come to- Pkthodon, we emphasize the fact that all gether, there is a narrow zone of intergra- species are completely terrestrial, with di- dation, beyond which northeastward to the rect development on land, and thus do not limits of the distribution of P. jordani, a migrate to and f?om water. maximum distance of 375 km, no speci- mens can be found with any red on their METHODS cheeks. Thus, the change between the two The two zones of intergradation were ob- forms is relatively abrupt. The other zone served as exercises,for undergraduate class- involves P. jordani and P. gh&wsu.s in the es. Two or three classes of 15 to 18 students Nantahala Mountains; one of five restricted visited the field sites each year. Most visits areas in which hybrids between these two were in September; one was in June; and species have been found. No hybridization’ threewereinApril.Thecl.&eintheGreat has been detected in most areas of contact Smoky Mountains-Balsam Mountains area between these species throughout the south- was found in 1973 and studied three times ern Appala&ians @I&&ton and Henry, in 1973 and 1974, during which time the 1970; Peabody, 1978). It is the exceptional classiCcation of colors was established. Ob- nature of the hybridization that has caused servations started at Heintooga Overlook in most taxonomists to continue to’ recognize the Great Smoky Mountains’National Park them as separate species (Highton and Hen- and proceeded at odometer-measured two- ry, 1970; Peabody, 1978; Larson, 1984). mile intervals along the service road toward Highton (1983) has recognized the geo- Round Bottom Camping Area (Fig. 2). The graphic representative of gZutinosus in the observations were made between 20~00 and Nantahalas as P. teyahalee. We believe that 23:00 at night, when salamanders are active the designation is unjustified (Hairston, in above ground, so as to avoid disturbing the press). habitat by turning logs and stones. At each These two hybrid zones have been under stop, a class attempted to collect 10 or more our observation for .18 and 20 years, re- specimens of Pkthudon jordani (12 in 1990), spectively, and identical measurements have and with one or two of us then scored the been made at least twice annually for 16 amount of red pigment on the cheeks of years. Other studies of the two species dur- each specimen. We recognized five catego- ing the same period have given us further ries of color. 0 for none, 1 for small spots, information about population density, pop- 2 for a continuous line along the top of the 932 NELSON G. HAIRSTON, SR ET AL. TABLE 1. Proportional distribution of color of Pfeth- odon jordani along the Heintooga-Round Bottom Road. Data from all field trips combined. Distance fium Hcintocga (miles) 0 2 4 6 N: 366 373 450 470 Amount of red: 0 0.997 0.86 1 0.120 0.006 : 0.003 0.019 0.121 0.324 0.278 0.215 0.072 3 0.238 0.402 4 0.040 0.304 collected and scored in the same way as at Heintooga, except that two scores for col- oration were recorded for each specimen. White spots, a characteristic of P. glutino- su.s, were recorded as 0 (none), 1 (very few spots along the sides or cheeks), 2 (spots FIG. 2A. Contour map of the vicinity of Heintooga. along the sides but none on the back), and Contour interval 1,000’ (305 m). Brolccn line outlines 3 (many spots on the sides and at least a the hybrid zone.’ In this area, Pkfhodon jorduni has few on the back). Red legs, a characteristic not been found below 4,000’ (1,220 m) and is uncom- P. jordani in the Nantahalas, were mon below 4,500’ (1,372 m), thus limiting the length of re- of the hybrid zone. O+n circle: Round Bottom; filled corded as 0 (none), 1 (very small spots of circles: cokcting stopq’x major peak 2B. Profile of red on the forelegs), 2 (red on the forelegs theareaasviewedfromthewestverticalexaggeration but little ifany on the hindlegs), and 3 (the 6: 1. Hatched area: hybrid zonq dotted line: approxi- proximal parts ofboth forelegs and hindlegs mate lower limit of P. jordani. fdly or mostly red). During each visit, stops cheek but little else, 3 if the whole cheek were made according to elevation.
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