A Foreword

Abstract

Sympatric congeneric anurans exist together under systems of adjustment to co-existence of their populations, systems that reflect a past history of the populations under allopatry and that also reflect interspecific interactions under subsequent sympatry. Sympatric species may affect one another through mismating even when genetic interchange is barred by hybrid sterility or hybrid inviability. Several isolating mechanisms usually operate to restrict hybridization of any given pair of sympatric anurans, with difference in mating call usually of paramount importance. Some important parameters of the call (viz., frequency and pulse rate) are usually functions of body size, and hence differentiation in these attributes may be a secondary effect of adaptive differentiation in body size. Other parameters of the call appear to have been the subject of selection per se. Ecological isolating mechanisms may represent side-products of adaptation to different ways of life in allopatric populations. Post-mating mechanisms of hybrid inviability or sterility reflect different developmental systems which are not necessarily the genetic systems involved in production of the phenotypic characters used in morphological classification. Examination of various allopatric and sympatric species pairs in the anurans indicates that, as known at present, there is no predictable sequence in which the various kinds of isolating mechanisms evolve. However, sympatric species always differ in mating call, and some allopatric populations have been shown to differ enough for the females to discriminate between the calls of their own and foreign males. In general, genetic incompatibility seems to be the last isolating mechanism to evolve. Some sympatric species are interfertile, and some allopatric species are intersterile. Greater differentiation of mating call in the overlap zone of species pairs than in non-overlap areas is taken as evidence for reinforcement of a difference in call through limited hybridization and selection against the hybridizers.