Growth and Development of Larval Rana temporaria: Local Variation and Countergradient Selection

—I raised tadpoles of the Common Frog (Rana temporaria) from populations in eight source ponds in southern Sweden, in a common garden experiment at two densities. Tadpoles from different populations differed in development rate; those from source ponds with high tadpole densities developed faster than those from less crowded ponds. Thus, differences among ponds in tadpole performance, which were documented in previous field studies, must have a genetic or maternal component. This result of source pond crowding likely resulted from microevolution and is an example of countergradient selection. In contrast, I found no significant effect of source pond hydroperiod; tadpoles from temporary ponds grew and developed at a rate similar to those from permanent ponds. Tadpoles of R. temporaria can respond plastically to pond drying by increasing development rate. I suggest adaptive plasticity in development rate decreased selection by pond drying in natural ponds. Local character variation is found in many species and arises from direct effects of environment on the phenotype or from underlying genetic variation. Phenotypic variation may be adaptive, which is usually referred to as adaptive plasticity (Via et al., 1995; Pigliucci, 1996), or nonadaptive, having the character of environmental modulation (sensu Smith-Gill, 1983). Similarly, genotypic variation may be adaptive (an effect of microevolution) or the result of nonadaptive processes (i.e., genetic drift or founder effect). In amphibians, two major causes of variation are those related directly to pond quality and those related to differences among eggs from which larvae hatched. The latter includes both genetic and maternal effects, such as egg size (Rossiter, 1996; Bernardo, 1996; Loman, 2002a). In southern Sweden, growth and development of Common Frog (Rana temporaria) tadpoles, differ among ponds (Loman, 2002b). Ponds studied had several environmental gradients that could influence tadpole growth and development rate, including pond hydroperiod. In the study area (and in other parts of its distribution), Common Frogs frequently breed in ponds that regularly dry before or during metamorphosis (Cooke 1985; Kutenkov and Panarin, 1995; Loman, 1996, 2002b). This may cause catastrophic mortality, but tadpoles of Common Frogs partly counter this (at least under laboratory conditions) by accelerating development when subject to cues suggesting pond drying (Loman, 1999; Merilä et al., 2000a). Another gradient was tadpole density, which varied considerably among ponds (Loman, 1996, 2002c). At high densities, tadpoles tend to develop more slowly and metamorphose later than those growing under low densities (Wilbur, 1977; Loman, 1999; Brady and Griffiths, 2000). Recently, cogradient and countergradient selection (Fig. 1) have attracted renewed interest from ecologists studying character variation (Berven et al. 1979, Conover and Schultz, 1995). Cogradient selection (Fig. 1A) occurs when selection pushes the reaction norm in the same direction as the trait expression that is plastically induced in that environment. For example, when fish were present, salamander (Ambystoma barbouri) larvae reduced activity, thus reducing feeding rate (Sih et al., 1992). Under standard laboratory conditions, reduced activity and feeding were most pronounced for larvae from populations at sites with fish; presumably these SHORTER COMMUNICATIONS 595