A Cretaceous divergence time between pelobatid frogs (Pelobates and Scaphiopus) : immunological studies of serum albumin

Serum albumin was purified from the Old World spadefoot toad Pelobates cultripes and from the New World spadefoot toads Scaphiopus hammondi and Scapliopus couchi and injected into rabbits. The resulting antisera were used in the quantitative micro‐complement fixation test to assess the degree of genetic relatedness between Pelobates and Scaphiopus as well as among six different species of Scaphiopus . Although Pelobates and Scaphiopus are morphologically and ecologically similar, and are considered to be close relatives, our immunological data suggest a divergence time of about 110 million years B.P. The albumins of the four species within the subgenus Spea ( S . hammondi, S . intermontamus , S . bombifrons, and S . multiplicatus ) were very similar. Albumins of the subgenus Scaphiopus ( S . couchi and S . holbrooki ), in turn, were more similar to each other than either was to S . hammondi . The intra‐ Scaphiopus results are consistent with previously reported electrophoretic studies, and a high correlation is observed between albumin immunological and electrophoretic genetic distances. These data and results from other biochemical studies provide no support for a single biogeographic model to explain the Holarctic distributions of amphibians.