GEOGRAPHIC AND REPRODUCTIVE ISOLATION IN ATLANTIC AND PACIFIC POPULATIONS OF PANAMANIAN BATHYGOBIUS.

Abstract

Thoughout most of the Tertiary the Atlantic and Pacific oceans were continuous and connected over much of the area that is now Central America and the northwestern corner of South America. Presumably populations of many marine species were also continuous in this range. The rise of the Isthmus of Panama 2-5 million years ago (Woodring, 1966) split this vast biota into two disjunct populations, the western Atlantic and the eastern Pacific. The land bridge has remained as a geographic barrier between the oceans and as such has provided a unique opportunity to study the evolution of many marine organisms. The Panama Canal has had little effect on the distribution of most groups because of the fresh water of Gatun Lake. Few marine species can survive a canal transit. (Concerning fishes see Hildebrand, 1939; R. W. Rubinoff and I. Rubinoff, 1968 and 1969). The species that were separated have continued to evolve independently and diverge from each other with time. Some groups are still morphologically identical on both coasts. Others have changed radically, probably in response to different selective pressures. These pairs of populations, with an obvious common origin, have been called geminate. It seems likely that some of the geminate populations have evolved potential reproductive isolating mechanisms. But since the presence of reproductive isolation is not necessarily correlated with morphological divergence, it would be difficult to predict which pairs are still conspecific. The Isthmus of of Panama is, both historically and logistically (the oceans are only 50 miles apart), ideally suited to an experimental laboratory study of isolating mechanisms. Most studies of isolating mechanisms have involved field work on species that are already sympatric. Some experimental studies on allopatric populations have been conducted with a limited number of terrestrial and fresh water animals (Littlejohn, 1969), but marine species have been largely ignored.