Remarks on the Geographical Distribution and Phyletic Trends of South American Toads

Abstract

The genus Bufo, widely spread over the South American continent, is absent only in a few of the Austral Patagonian territories, in Tierra del Fuego, and on some of the Pacific coastal deserts. It appears in limited numbers in the highest Andean areas. Due to their ecological plasticity, these toads have been able to fit favorably into a great variety of environmental niches; their present trends of distribution are in accordance with the general patterns of their phylogeny and the present physiognomy of their speciation. This paper will deal with some aspects of the geographical distribution of the most important lines of the neotropical toads, bearing in mind, moreover, some of the recent contributions on the systematics and ecology of several of these forms, together with their genetic and physiological relationships. Disregarding any previous taxonomic or historical question, a discussion of infrageneric relationships in the neotropical forms could be pointed out from Tihen’s paper (1962) which deals especially with comparative osteological features. Although the studies of Noble (1931) and Davis (1936) deal with the phylogenetic relations of the Bufonidae, as suggested by any available anatomical or zoogeographic evidence, no special attempt had been made to determine and delimit the supraspecific evolutionary groupings of this polymorphous genus before reports by Blair (1963) concerning the origins and evolutionary development of the actual species of toads, particularly of the New World species. Among the most interesting conclusions of Blair’s experimental contributions is that two main lines of American Bufo have been stressed, based on the ancient evolutionary and biogeographical history of the most precocious stocks of these Anurans. A northern line, related to some Tertiary calamita-like stock, could support the present relationships between the Nearctic groups: B. boreas-punctatus and B. americanus-woodhousei. A hypothetical relationship between them and the neotropical spinulosus complex was also suggested. On the other hand, a southern line, as an early segregation from a melanostictus-like stock has been postulated due to the present evolutionary trends of groups such as valliceps and allied forms, canaliferus, coccifer, occidentalis, etc. Morphological and biological affinities of these toads with the widespread neotropical marinus group were stressed. Such a tentative arrangement must be considered seriously from every approach regarding the general distribution and ecology of neotropical bu- fonids. Moreover, it must be pointed out that assumptions taken from Blair’s analysis are supported by objective evidence from genetic compatibility and experiments on interspecific hybridization.