is filled by many organisms on the continents but in early Hawaii was almost empty.

As one species diversifies into many, a variety of different evolutionary paths can be taken (see Figure 11). An ancestral species can give rise to a daughter species while remaining relatively unchanged itself. Or a succession of single species can lead from an ancestral species to a single current species. Or an ancestral species can undergo repeated divisions, producing complex networks of evolutionary relationships (Panel 2).

The speciation of drosophilid flies in Hawaii is continuing to occur. For example, a species known as Drosophila silvestris occupies several discrete patches of forest on the Big Island (see Figures 12a and 12b), living in cool, wet forests above 750 meters (2,500 feet) in elevation and laying its eggs in the decaying bark of trees. Males of D. silvestris have a series of hairs on their forelegs that they brush against females during courtship. On the northeastern half of the island (known as the Hilo side), the males have many more of these hairs than do the males on the southwestern side (the Kona side). These two populations are developing physical and behavioral differences that over time might split a single species into separate species.

Figure 12a

Two males of the species D. silvestris grapple head to head. (Photograph courtesy of Kevin Kaneshiro.)

Figure 12b

Map of the Big Island shows five regions inhabited by populations belonging to the species D. silvestris (shaded in gray). The populations on the Hilo side of the island, which are younger than the Kona side populations, are developing differences that over time could lead to the origin of a new species. (Map adapted from Hampton L. Carson, “Sexual Selection: A Driver of Genetic Change in Hawaiian Drosophila,” Journal of Heredity 88:343-352, 1997. The contours are in meters.)