Three flies and three islands: parallel evolution in Drosophila.

Most organisms are evolutionary conservatives; they may subdivide their niches but tend to remain within them. Yet the fossil record shows many cases of breakthrough to a new mode of life. How may such evolutionary innovation be recognized at this time level? Three species of Drosophila have accomplished an innovation in that they breed as obligate commensals on tropical land crabs. This could be dismissed as a curious aberration were it not for the fact that the three flies concerned represent three different phyletic lines of the family. Although Drosophila is abundant on the continents, these three parallel innovative evolutions have occurred on islands. The proposal is made that the genetic systems of many conservative groups of organisms carry variability that would permit them to evolve in a novel direction. The realization of this capacity, however, is possible only under special environmental conditions.