Watching a New Species Develop?

Before Darwin, scientists generally agreed that all species of life had existed since creation and would continue unchanged to the end of the world. After Darwin theorized on the origin of species in 1859 it was soon acknowledged that new species do evolve from older ones in a gradual process. Possible beginnings of new species have since been recognized, catalogued and thoroughly investigated, but geneticists still remain ignorant of just what mechanisms trigger them. Now there is hope that a new thread of inquiry might begin to unravel the mystery. A new species appears to be developing in a laboratory.

It happened quite accidentally. Dr. Theodosius Dobzhansky, one of the world's foremost genetic researchers, was experimenting with an amber-colored South American fruit fly known as Drosophila paulistorum at Manhattan's Rockefeller University. He was interested in one particular strain of the fly from the Llanos district of Colombia, and he isolated it from other strains in 1958. Five years later he and his assistant, Dr. Olga Pavlovsky, routinely attempted to interbreed the Llanos flies with similar strains and observe the results of the mating.

A species is defined by scientists as a group that can reproduce itself. Two species are marked as distinct if mixed members either will not mate or cannot produce fertile male and female offspring. Thus the horse and the donkey are separate species because their issue, the mule, is always sterile. In the case of the fruit flies, the Llanos strain was capable of producing fertile young with other strains in 1958, but when they were remated later, the males of the isolated group could not sire fertile sons with any females except those of their own strain.

"It was a complete surprise," says Dobzhansky. "And it happened right under our noses." But he does not yet know why. One tentative theory now being investigated is that the isolated flies may have been infected in some way that caused sterility with other strains. In any case, the new species is only in an incipient stage. The "new Llanos" females, when mated with males of the related strains, can still bear fertile female offspring. Until they no longer can, the species will not be fully distinct. A new generation is born every two weeks, and Dr. Dobzhansky and his colleagues will be closely watching to see how the species evolves.

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