Inbreeding is generally frowned upon, even in the animal kingdom. Besides being awkward, it decreases genetic diversity and increases the frequency of individuals with genetic diseases in a population (kind of like that one episode of the X-Files). But, that doesn't mean it's all bad. If acted upon by natural selection, bad mutations can be wiped out more easily, thus speeding the rate of evolution. And for some reason, third- and fourth-cousins that marry each other in Iceland have more children.

Similar to the people who live in Iceland, fruit flies may receive some benefit from inbreeding. Researchers in Europe gave female fruit flies a simple choice: Mate with your brother or a non-brother. They also had females choose between their father or a non-father. Similarly, males were given the choice of mating with their sister (vs. non-sister) or mother (vs. non-mother). The results are shown in the graph below.

When given a choice between a brother or a non-brother, females preferred mating with their brother. But, males did not prefer their sister. They simply tolerated mating with their sister, picking them roughly half the time. Similarly, females showed no preference when choosing between their father or a non-father, picking their dad about 50% of the time. Males wooed their moms, but no successful mating occurred. Apparently, mom knows best. (Or, more likely, the mothers were unable to mate due to age or refractory period.)

The authors conclude that inbreeding occurs in fruit flies because it may increase their inclusive fitness. Whatever the reason, it's still gross.

Source: Loyau A, Cornuau JH, Clobert J, Danchin É (2012). "Incestuous Sisters: Mate Preference for Brothers over Unrelated Males in Drosophila melanogaster." PLoS ONE 7(12): e51293. doi:10.1371/journal.pone.0051293