EXPERIMENTAL-EVIDENCE FOR HABITAT DEPENDENT SELECTION IN A BOMBINA HYBRID ZONE

Hybridizing taxa remain distinct for two main reasons. Natural selecti on acts against hybrids either because of their incompatible genome, o r because of differential adaptation of the pure types across an envir onmental gradient. Here, we provide experimental evidence that the loc ation of the Bombina (Anura: Discoglossidae) hybrid zone in Croatia is , at least in part, determined by differential adaptation. B. bombina typically breeds in permanent water in the lowland, whereas B. variega ta reproduces in puddles at higher elevations. In a reciprocal translo cation, pure bombina and variegata tadpoles were introduced in equal p roportions into lowland pond enclosures and upland puddles. After thre e weeks, variegata exceeded bombina in survival and growth in both hab itats. The effect was most pronounced in puddles, where the few surviv ing bombina tadpoles had hardly grown at all. In comparison to variega ta, the smaller hatchlings of bombina grew relatively faster in ponds, but remained smaller in absolute terms. Nevertheless, B. bombina appe ars better adapted to ponds than to puddles. The mechanisms by which v ariegata is excluded from ponds remain to be demonstrated. These data show that habitat dependent selection prevents the invasion of bombina tadpole traits into the variegata gene pool. Given the strong linkage disequilibria in hybrid populations, differential selection on tadpol es may be sufficient to maintain the integrity of the two gene pools.