Dynamic Change of a Zone of Parapatry between Two Species of Pholidobolus (Sauria: Gymnophthalmidae)

cently on groups of closely-related species that are distributed parapatrically (for a recent review see Barton and Hewitt, 1980). Parapatrically distributed organisms usually meet at narrow hybrid zones, although in some cases hybridization may be rare or unobserved (Montanucci, 1973; Selander et al., 1974; Nevo et al., 1974; Caire and Zimmerman, 1975; Greenbaum and Baker, 1976; Hillis et al., 1983). These zones of sympatry often occur along ecotones and are generally considered to be relatively stable, especially if selection against hybridization is strong (Barton, 1979). Montanucci's (1973) systematic review of lizards of the genus Pholidobolus revealed that the five species are distributed parapatrically in the Andes of Ecuador (Fig. 1). Areas of sympatry are extremely limited; overlap occurs only between the ranges of three species-pairs (Fig. 1): (1) P. montium and P. affinis in the upper end of the Latacunga Basin (Hoya del Rio Cutuchi) in the vicinity of Mulal6; (2) P. affinis and P. prefrontalis at the edge of the Palmira Desert; and (3) P. prefrontalis and P. macbrydei on the east side of the Cuenca Valley (Hoya del Rio Paute). All of these zones are in ecotonal situations.