The drosophilid flies of Hawaii offer one of the best examples in the world of an adaptive radiation, where an ancestral species has given rise to hundreds of new species. In particular, evolutionary biologists have focused their attention on a group of about 100 drosophilid species that have characteristic light and dark 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.
Drosophila has long been a favorite model organism for geneticists, in part because the chromosomes in some of their cells condense into large, thick structures that are easily visible through a microscope. Biologists first capture the flies in their natural habitats and bring them into the laboratory. There the female flies lay eggs that are grown to the larval stage. At that point, the salivary glands of the larvae are extracted, and chromosomes from the salivary cells are stained and mounted on slides. When viewed with a microscope, these so-called polytene chromsomes display hundreds of alternating dark and light bands of differing sizes.
Using polytene chromosomes, it is especially easy to detect a kind of chromosomal rearrangement called an inversion. Sometimes a chromosome can be damaged at two separate places. When molecular mechanisms within the cell seek to repair the damage, a segment of the chromosome can be flipped with no effect on the functioning of the chromosome. The result is a rearranged chromosome in which a section of the chromosome, with its characteristic light and dark bands, has a reversed orientation.