Albumin Evolution in Frogs: A Test of the Evolutionary Clock Hypothesis

Abstract

Frogs are an ancient group compared to placental mammals. Yet, although there are about as many species of frogs as there are of mammals, zoologists consider that frogs have undergone only limited morphological divergence, while placental mammals have diversified greatly in morphology and way of life. The serum albumins of numerous frog species were compared by the quantitative microcomplement fixation technique. Frogs that are morphologically similar enough to merit taxonomic distinction at only the species level often exhibit differences in the serological properties of their albumins larger than those usually seen between mammals placed in distinct families or suborders. Thus, there seems to be a contrast between albumin evolution and evolution at the organismal level. The large differences between albumins among frogs can be explained by the hypothesis that albumin evolution has proceeded at the same rate in frogs as in mammals.The hypothesis that serum albumin behaves as an evolutionary clock was proposed in 1967 (1, 2) to account for the results of immunological comparisons of albumins from a wide variety of primate species. Albumin appeared to have been evolving at about the same rate in all primate lineages tested, including the human lineage. More recent immunological studies of primate albumins substantiate this hypothesis (3). A similar rate of evolutionary change, approximately 100 units of immunological distance per 60 million years, has also been detected in the albumins of other mammals (4), as well as of lizards and crocodilians (5). Evidently, the rate of fixation of mutations causing amino-acid substitutions is approximately constant for albumin regardless of the species, mammal or reptile, in which it occurs. A similar phenomenon has been encountered by those who have studied the evolution of other proteins, for example, haemoglobin (6).We thought it important to determine whether a similar rate of albumin evolution occurred in frogs, because although frogs are an ancient group their morphological uniformity makes them the most easily diagnosed of all vertebrate groups. Thus, zoologists place all living frogs in the single order Anura, even though several of the lineages leading to modern frogs probably diverged from each other about 150 million years ago (7-12). By contrast, the placental mammals, for example, are divided into 16 orders because of their great morphological diversity. Yet the diversification of extant placental lineages began only about 75 million years ago (13). The clock hypothesis predicts that we should find enormous albumin differences between distantly related frogs, far larger than those within or between orders of placental mammals.If, on the other hand, morphological resemblance were used as a basis for prediction, we should expect to find moderate albumin differences among frogs. A comparative immunological study of frog albumins has, accordingly, been performed. MATERIALS AND METHODSSerum samples were obtained from representatives of 26 species belonging to ...

A Preliminary Phylogenetic Analysis of Metapenaeopsis (Decapoda: Penaeidae) Based on Mitochondrial DNA Sequences of Selected Species from the Indo-West Pacific

“…However, this does not necessarily mean that there is no genetic heterogeneity between taxa, as the rate of evolution of morphological features may be different from that of molecular evolution (Wallace et al, 1971;Wilson et al, 1977). The present data show 98% similarity at the amino acid sequences of COI among Metapenaeopsis spp.…”

The Importance of Gene Rearrangement in Evolution: Evidence from Studies on Rates of Chromosomal, Protein, and Anatomical Evolution

“…Nonetheless, despite the vast effort devoted in recent years to the study of nucleic acid and protein evolution, serious problems arise when one tries to reconcile organismal evolution with macromolecular evolution (1)(2)(3). We now present evidence that there may be a close parallel between chromosomal evolution and organismal evolution.…”

Two Types of Molecular Evolution. Evidence from Studies of Interspecific Hybridization

“…We propose that evolutionary change in regulatory systems accounts for evolution at and beyond the anatomical level. This proposal emerges from attempts to explain the observation that protein evolution and anatomical evolution can proceed independently (1)(2)(3). This independence is illustrated by protein and anatomical studies on frogs and mammals.…”