Factors Affecting Secondary Intergradation Between Two Allopatric Populations in the Bufo Woodhousei Complex

Abstract

There has been a marked lack of specific information regarding the distributional, taxonomic, and genetic relationships of the toads, Bufo woodhousei and Bufo fowleri, along the line of contact of their ranges in the central United States. In an effort to elucidate these relationships, field and laboratory studies of eastern and central Texas populations were conducted over a five-year period. A total of 932 adults were collected in the field and forty-one laboratory crosses and backcrosses were made involving the parental forms and their laboratory hybrids in all possible combinations. Special attention was given in the field to an evaluation of the commonly observed anuran isolating mechanisms as they affected Bufo woodhousei and B. fowleri along the contact zone. Evidence is presented indicating that ecological isolation is the principal factor acting to maintain the integrity of the two forms in Texas. Even this was found to be relatively ineffective in those areas of mixed or interdigitated habitat resulting from man's disruption of the normal ecological balance. The morphological relationships of the parental forms to each other and to the intergrade progeny were investigated through statistical treatment of six body measurements. These showed the intergrades to be generally intermediate between the parental taxa. A correlation was found to exist in the morphological nature of an intergrade sample and its geographical position within the intergrade zone. It was concluded that B. woodhousei and B. fowleri are intergrading freely over a narrow but extended north-south zone in Texas and probably should be regarded as subspecies. It was further concluded that the present contact is a secondary one and is explainable on the basis of fragmentation of a formerly homogeneous population during the Pleistocene.