The geological association of the Greater Antilles with North and South America in the late Cretaceous led to the hypothesis that the present Antillean biota reflects those ancient land connections. Molecular data from diverse West Indian amphibians and reptiles and their mainland relatives support a more recent derivation of the Antillean vertebrate fauna by overwater dispersal. The catastrophic bolide impact in the Caribbean region at the close of the Cretaceous provides a proximate cause for the absence of an ancient West Indian biota.

AB STRACTTo assess the significance of macromolecular sequence differences among species, we compared the serum albumins of 81 pairs of vertebrate species capable of producing viable hybrids. Micro-complement fixation experiments showed that the average difference between the albumins within such pairs was only 3 immunological distance units for placental mammals (31 pairs), but 36 units for frogs (50 pairs We think the most likely explanation for the marked molecular restriction on hybridization among mammals is that the ratio of regulatory evolution to protein evolution is higher for mammals than for frogs. Mammals may have experienced unusually rapid regulatory evolution; indeed, this could be the factor responsible for their unusually rapid anatomical evolution.There may be two major types of molecular evolution. One is the process of protein evolution, which goes on at about the same rate in all species. The other is a process whose rate is variable and which is responsible for evolutionary changes in anatomy and' way of life. 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 (1-3). This independence is illustrated by protein and anatomical studies on frogs and mammals. Frogs (Anura) are an ancient group that has undergone much protein evolution (1, 2, 4-7) but little anatomical evolution during its 150-million-year history. Although there are thousands of frog species living today, they are all rather alike in anatomy and way of life. By contrast, the placental mammals, which are only 75 million years old, have undergone extensive anatomical evolution. The diversity in anatomy and way of life represented by bats, whales, sloths, and people is unparalleled among frogs. Yet placental mammals have experienced less protein evolution than frogs have. While the rate of protein evolution is similar in the two groups, their rates of anatomical evolution differ greatly (1, -2). This remarkable contrast between protein evolution and anatomical evolution implies that protein evolution may not be at the basis of anatomical evolution.For the idea that evolutionary changes in regulatory systems may provide the basis for anatomical evolution, we are indebted to Wolpert (8), Britten and Davidson (9, 10), and above all, Ohno (11,12

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We have compared the relative rates of protein evolution and chromosomal evolution in frogs and mammals. The average rate of change in chromosome number has been about 20 times faster in mammals than in frogs. Whereas it takes only 3.5 million years, on the average, for a pair of mammal species to develop a difference in chromosome number, the corresponding period for frogs is 70 million years. In contrast, the rate of protein evolution in mammals has been roughly equal to that in frogs. The rapid rate of gene rearrangement in mammals parallels both their rapid anatomical evolution and their rapid evolutionary loss of the potential for interspecific hybridization. Thus, gene rearrangements may be more important than point mutations as sources for evolutionary changes in anatomy and way of life.

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