THE DYNAMICS OF TWO HYBRID ZONES IN APPALACHIAN SALAMANDERS OF THE GENUSPLETHODON

Abstract

Two zones of intergradation between populations of Plethodon 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 toward the Smokies, there is increasing frequency 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 failure 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 intergrading forms, which most likely took place after the Hypsithermal Interval, 8,000-5,000 BP. At Coweeta Hydrologic Laboratory in the Nantahala Mountains, Plethodon jordani and P. glutinosus hybridize at intermediate elevations. The lateral white spots of glutinosus decrease and the red on the legs of jordani increases with elevation from 685 m to 1,052 m. At the higher elevation, the proportion of animals scored as "pure" jordani declined significantly from 1974 to 1990, an indication that the hybrid zone is spreading upward. The rate of spread is too great to be explained by random diffusion, so selection for glutinosus characters is the best explanation. 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 disturbances have caused hybridization in other organisms.

Islands in the sky or squeezed at the top? Ecological causes of elevational range limits in montane salamanders

“…Conversely, species in clades that diversifi ed in association with warm climates (e.g. tropical or subtropical conditions) should generally follow the elevational distribution of P. jordani (Hairston et al 1992, Hairston and Wiley 1993, Chatfi eld et al 2010.…”

Plethodontid Salamander Response to Silvicultural Practices in Missouri Ozark Forests

“…We see evidence of this in the significantly higher densities of salamanders on second-growth sites that had undergone regeneration cutting 70 to 80 years earlier. The rate of salamanders returning to harvested areas is slow, however, perhaps because of long generation times, slow dispersal rates, and high site fidelity (Hairston 1983;Hairston et al 1992), coupled with the slow recovery rate of the vegetation on a site after harvesting. After a site has been clearcut, it is subject to greater daily fluctuations in temperature and humidity levels and higher wind speeds than a closed-canopy forest.…”

Quantifying the spatiotemporal dynamics in a chorus frog (Pseudacris) hybrid zone over 30 years

“…He proposed that hybrids may not be "at an absolute disadvantage to parental types" here as they were throughout the rest of the parental range and that this intermediate zone may be stable for an extended time (Gartside 1980). A change in exogenous selection could be expected to alter dispersal-independent hybrid zones if the environment that favors hybrids expands and allows hybrids to expand their range (Hairston et al 1992). For example, Hairston et al (1992) demonstrate movement of a hybrid cline between salamanders (genus Plethodon) and propose a recent selective advantage for traits of one species over the other, driven by human modifications to the environment.…”

“…A change in exogenous selection could be expected to alter dispersal-independent hybrid zones if the environment that favors hybrids expands and allows hybrids to expand their range (Hairston et al 1992). For example, Hairston et al (1992) demonstrate movement of a hybrid cline between salamanders (genus Plethodon) and propose a recent selective advantage for traits of one species over the other, driven by human modifications to the environment. In dispersal-dependent (tension) zones, however, movement may be expected either when shifts occur in parental density and dispersal or when endogenous selection pressure on hybrids changes (Barton and Hewitt 1985;Buggs 2007;Carling and Zuckerberg 2011;Smith et al 2013b).…”

“…They are thought to reach equilibrium in areas of low population density but may move if parental ranges shift (Barton and Hewitt 1985;Carling and Zuckerberg 2011;Smith et al 2013a;Taylor et al 2015). Theory suggests that clines of either model can move in response to factors such as environmental or climatic change (Hairston et al 1992;Parmesan et al 1999;Britch et al 2001;De La Torre et al 2015;Taylor et al 2015), dominance drive (a dominant allele replacing a recessive allele; Mallet 1986), or asymmetrical hybridization (Bronson et al 2003), but relatively few long-term studies have successfully and directly documented significant movement in clines of either model (Buggs 2007;Carling and Zuckerberg 2011).…”