drosophilids in Hawaii has led scientists to a remarkable conclusion. All of the native Drosophila and Scaptomyza species in Hawaii appear to be descended from a single ancestral species that colonized the islands millions of years ago! In fact, all of the approximately 800 species of drosophilids in Hawaii could be descended from a single fertilized fly that somehow reached the islands—perhaps blown there by a storm, or carried to the islands in a scrap of fruit stuck to the feathers of a bird.

Since that time, the descendents of the original colonists have undergone what evolutionary biologists call an adaptive radiation. New species have evolved and have occupied a wide range of ecological niches. Several interacting factors have contributed to this adaptive radiation. An especially important factor for the Hawaiian drosophilids has been what is called the founder effect. Many new populations of drosophilids in Hawaii must have become established in much the same way as did the original population. A few individuals or a single fertilized female must have journeyed or been transported from one area of suitable habitat within an island to another such area, or from one island to another. These founders carried with them just a subset of the total genetic variability within its species. As a result, the physical characteristics and behaviors of the founders could differ from those typical of the parental population. Under such circumstances, a founder population can diverge from the ancestral population and eventually may become a new species.

The great ecological diversity of the Hawaiian islands also plays a role in adaptive radiations. Drosophila species continually expanded into wetter or drier areas, higher or lower elevations, and regions of differing vegetation. The members of a species able to survive in these new areas can acquire new adaptations that set them apart from the original species.

Finally, the lack of competitors in island settings can spur the evolution of new species. In Hawaii, the drosophilids could move to new islands or into ecological niches that on the continents would already have been filled by other species. For example, many Hawaiian drosophilids lay eggs in decaying leaves on the ground, an ecological niche that

Figure 11

An ancestral species that lived in the past can give rise to multiple species through a variety of different evolutionary pathways. An ancestral species can gradually evolve into a series of what would be considered new species while remaining a single genetically connected population (path 1). Or a species can remain unchanged for a long period of time (path 2). Or an ancestral species can undergo a series of splits, generating new species that in turn become extinct or undergo further speciation (path 3).



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