organisms: the fly species on earth far out-number all of the vertebrate species combined. But the native insects of the Hawaiian islands include very few separate fly genera, and most of the native fly species are drosophilids.
When biologists began to study the evolutionary history of the Hawaiian drosophilids, they first examined the physical similarities and differences of the species. If two species have very similar appearances, scientists might hypothesize that both are descended from an ancestral species that lived quite recently. If two species are physically quite distinct, scientists could infer that they are more distantly related. Researchers then would seek additional evidence to support or reject these hypotheses. For example, two species can develop similar adaptations if they live in similar environments and therefore can appear to be more closely related than they actually are.
In recent decades, biologists have gained an additional way of examining the relationships among species. Each individual fly has a particular sequence of the chemical units that make up the DNA in its cells. In general, these sequences are more similar among the members of a single species than they are between the members of different species. Similarly, DNA sequences generally are more similar between closely related species than they are between more distantly related species. Genetic sequences accumulate changes over the generations as DNA randomly mutates and is influenced by natural selection or other evolutionary processes. If the DNA sequences of two Drosophila species are more similar, the two species are more likely to be descended from a relatively recent ancestral species, because their DNA has not had much time to diverge. If the DNA sequences are less similar, the two species had more time to accumulate genetic changes, indicating that their common ancestral species lived in the more distant past.
Study of the physical and genetic differences among the hundreds of species of native