Many such mutations are lost when a chromosome with a new inversion is not passed on to offspring. But in other cases many descendants of the fly in which the mutation first occurred inherit the chromosome with the inversion, and in some cases the structurally modified chromosome will spread with each new generation until it is found in all the members of a given species. In this way, different Drosophila species come to have different genetic banding patterns. Their chromosomes thus act as a sort of genetic bar code, allowing separate species to be compared.
Numbers in arrows indicate the number of founder events proposed to explain the origin of the picture-winged drosophilids on each island group. The number of picture-winged species found on each island is given in parentheses. (Diagram adapted from Hampton L. Carson, “Chromosomal Sequences and Inter-Island Colonizations in Hawaiian Drosophila,” Genetics 103:465-482, 1983.)
Beginning with the work of University of Hawaii geneticist Hampton Carson in the 1960s, researchers have used these banding patterns to work out the evolutionary history of the picture-winged drosophilids. If two species are descended from a single ancestral species, they will each inherit the bar code of their ancestor. At the same time, one of the daughter species may carry a new inversion that distinguishes it from its sister species, and it will pass this new marker on to any species that descend from it.
By studying these chromosomal banding patterns, biologists have reconstructed the sequence in which existing species of drosophilids moved from older islands to newer islands and formed new species. For example, the Big Island of Hawaii currently has 26 species of picture-winged drosophilids. Most of these species resulted from 15 founder events from the Maui complex of islands. In each of these events, a small group of flies (or perhaps a single fertilized female fly) journeyed from Maui or its surrounding islands to the Big Island. These founder events produced one or more drosophilid species that became distinct from the ancestral species they left behind. The remaining picture-winged species of the Big Island appear to result from three founder events from Oahu and one from Kauai. All of these species on the Big Island formed within the past half million years and are found nowhere else in the world.