The Picture-Winged Drosophilids
The drosophilid flies of Hawaii provide an excellent example of “adaptive radiation,” in which an ancestral species gives rise to a very large number of new species in a relatively short time. Evolutionary biologists have focused particular attention on a group of about 100 drosophilid species that have characteristic pigmented markings on their large wings. Known as the picture-winged drosophilids, these species carry within them a remarkable biological record of the group’s evolutionary history.
[Chromosome: A double stranded DNA molecule that contains a series of specific genes along its length. In most sexually reproducing organisms, chromosomes occur in pairs, with one member of the pair being inherited from each parent.]
Cells in the salivary glands of all Drosophila larvae contain special chromosomal structures known as polytene chromosomes. Easily visible through a microscope, these polytene chromosomes display hundreds of alternating dark and light bands of different sizes. These
Photograph of a polytene chromosome from a Drosophila larva shows two breakpoints (indicated by solid bars) where a portion of the chromosome is inverted compared to the same chromosome in other species.
banding patterns make it especially easy to detect a kind of chromosomal rearrangement known as an inversion. Sometimes, a mistake during the duplication of DNA can cause a segment of the chromosome to be flipped. The result is a rearranged chromosome in which a section of the chromosome, with its characteristic light and dark bands, has a reversed orientation. Many inversions of this type have occurred in different segments of chromosomes in different species of flies.
As individual species of drosophilids on the Hawaiian islands have diversified to form multiple species, researchers have used the resulting changes in banding patterns to reconstruct the sequence in which existing species of drosophilids moved from older islands to newer islands and gave rise to new species. For example, the “Big Island” of Hawaii, which is the youngest in the island chain, currently has 26 species of picture-winged drosophilids. By examining the specific chromosome inversions in these colonizing species and comparing them with species that live on islands that are older, researchers have determined that flies on the Big Island have all originated from 19 separate colonizations of the island by a small group of flies (or perhaps single fertilized female flies) from one of the older islands.