Genetically fixed characters and evolutionary divergence.

Abstract

Data from hybridization permit assessment of relationships of the toads (genus Bufo ) which occur on all continents except Australia and permit a crude estimate of the degree of genetic change with time. The evidence indicates that toads have been very conservative with respect to change. A major dichotomy into broad-skulled, warmth-adapted and narrow-skulled, cold-adapted types occurred in the New World 15 million or more years ago. The narrow-skulled toads have reached Africa at least twice. Hybridization between Eurasian and New World species shows that after separation for an estimated 10 million years some species remain genetically similar enough to produce viable F1 hybrids in both reciprocals of the cross. Broad-skulled toads appear to have been in Africa for at least 15 million years, as indicated by the fossil record, and in their main radiation they have changed their chromosomal number from 2n=22 to 2n=20. Nevertheless, their females form viable F1 hybrids with New World broad-skulled males after presumed isolation for more than 15 million years. Intermediate types live today in the New World connecting both skull types, and some cross easily with both. Evolutionary conservatism of Bufo with respect to genetic compatibility contrasts with the rapid change in Rana .