The drosophilid species of Hawaii are so different from each other that it is hard to believe they are all descended from a common ancestral species. But the biological evidence is compelling. All of the approximately 800 species of drosophilids on Hawaii are descended from the members of a single species of flies that made their way to the Hawaiian islands many millions of years ago. In fact, all of the Hawaiian drosophilids may be descended from a single fertilized female that reached the Hawaiian islands!

No one will ever know exactly how this fertilized fly traveled to the islands (if it was just one). Maybe it was blown to Hawaii in a storm. Maybe it was carried on a solid object to the islands. But somehow it survived the trip, and when it arrived at the islands it began to produce offspring.

Biologists suspected for a long time that the Hawaiian drosophilids were descended from a single ancestral species because of physical characteristics they share. But one piece of evidence has been particularly persuasive. When drosophilids are in their larval stage, biologists can examine cells from their very large salivary glands. These cells contain a special kind of chromosome called a polytene chromosome. In a polytene chromosome, many copies of a single DNA molecule line up side by side, making the chromosome large enough to see with a microscope. If a stain is added to the salivary tissue, the chromosomes exhibit a characteristic pattern of hundreds of light and dark bands along their lengths. The polytene chromosomes of the Hawaiian drosophilids, along with other genetic data, point toward a common origin for these very different species of flies.

Polytene chromosomes also can be used to trace the evolutionary history of individual drosophilid species. These chromosomes can reveal a particular kind of mutation called an inversion. Inversions are rare events that result in a section of a chromosome becoming reversed with respect to the original sequence. Inversions occur as a consequence of molecular damage and repair processes in the nucleus of the cell. Biologists can recognize these inversions in polytene chromosomes because the pattern of light and dark bands along part of the chromosome is reversed (see Figure 2).

These inversions provide a way of reconstructing evolutionary relationships. For example, if two species of picture-winged drosophilids both have the same inversion, they must be descended from an ancestral species that had that inversion. If one species has an inversion that the other does not, that inversion must have occurred in the time since the two species split from a common ancestor.

For example, think about the original fertilized fly that may have been the ancestor of all the Hawaiian drosophilids. As that fly began to produce offspring, they would have closely resembled the original colonist. Over time, this small population of flies would grow.

Eventually a group of flies must have become separated in some way from the others, either geographically, behaviorally, or ecologically. Females from this isolated population might have laid their eggs in different substances or become adapted to a different habitat. The members of this population

Figure 2

Polytene chromosomes from larvae of drosophilid flies provide a way to detect chromosomal inversions. In the chromosome shown here, the chromosomal segment between the two indicated points is inverted compared to the same chromosome in other species. (Photograph courtesy of Hampton L. Carson, based on an original photograph by Harrison D. Stalker, Washington University, St. Louis.)



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