Albumin Evolution and Organismal Evolution in Tree Frogs (Hylidae)

Maxson, L. R., and A. C. Wilson (Department of Biochemistry, University of California, Berkeley, California 94720) 1975. Albumin evolution and organismal evolution in tree frogs (Hylidae). Syst. Zool. 24:1-15.-To explore the relationship between organismal and protein evolution, a comparative study was made of the serum albumins of tree frogs (Hylidae). Attention was centered on those Holarctic hylines whose anatomy and way of life have beenintensively studied, namely species of Acris, Hyla, and Pseudacris. An albumin phylogeny was constructed from immunological distance data obtained with rabbit antisera to the purified albumins of 12 hyline species by means of the micro-complement fixation test. An analogous study with antisera to purified hemoglobin from 4 of these species was in approximate agreement with the albumin phylogeny. The albumin phylogeny revealed an apparent case of convergent morphological evolution involving H. wrightorum and H. regilla. These two species are similar enough in external morphology to have been classified in the same species by some recent authors. Yet their albumins differ as much from each other immunologically as from Acris albumin. This case is discussed more fully elsewhere (Maxson & Wilson, 1974). Discrepancies between organismal resemblance and albumin resemblance can also arise because of unequal rates of organismal divergence. Acris provides an example of this. The albumin phylogeny, supported by hemoglobin evidence, shows that the Acris lineage falls cladistically within the North American assemblage of Hyla lineages. Acris is more similar at the protein level to some Hyla species than the latter are to other Hyla species. Yet Acris is very different from Hyla in anatomy and way of life. The Acris lineage evidently underwent unusually rapid organismal evolution. However, there is no indication of accelerated albumin evolution in this lineage. Limnaoedus and Pseudacris provide additional examples of this sort. We infer that organismal and protein evolution can proceed at independent rates. This provides taxonomists with a dilemma: Should classifications be revised on the basis of protein data? The albumin results are consistent with recent ideas about the historical biogeography of hyline tree frogs. By assuming that albumin behaves as an evolutionary clock, which evolves at the rate of 1.7 immunological distance units per million years, it is calculated that a) Australian hylines diverged from New World hylines about 75 million years ago, b) North American hylines diverged from South American hylines 65 million years ago, and c) Eurasian hylines diverged from North American hylines about 40 million years ago. These estimated times are in approximate agreement with non-molecular evidence concerning the times when land connections existed between these continents. The assumed rate of albumin evolution is similar to that estimated elsewhere for such diverse organisms as mammals, iguanid lizards, crocodilians and ranoid frogs. The inference that albumin behaves approximately as an evolutionary clock is thus extended to include tree frogs. [Anura; albumins; immunology.] It is becoming increasingly apparent that organisms and their molecules can evolve at independent rates. Whereas organismal evolution has gone on far more slowly in most anuran lineages than in lineages of placental mammals, protein evolution has 1 Present address: Provisional Department of Genetics and Development, University of Illinois, Urbana, Illinois 61801. evidently proceeded equally rapidly in the two groups (Wallace, Maxson, and Wilson, 1971; Wallace, King and Wilson, 1973; Wilson, Maxson and Sarich, 1974; Wilson, Sarich and Maxson, 1974). Other examples of the independence of morphological and protein evolution are provided by the work of Mao and Dessauer (1971), Hight et al. (1974), Turner (1974), Nolan et al. (1975), and King and Wilson (1975). As