English: Mosquitofish (Gambusia affinis) on Andros Island in the Bahamas show high levels of allopatric speciation that has led to reproductive isolation. Populations have bred in small ponds called blue holes that have become geographically isolated. As a result, mosquitofish habitats with more predatory fish have been selected for a wider, streamlined body type that allows them to swim faster (1). In ponds with less predation, mosquito fish have evolved to have a slimmer body type that allows them to swim steadily for longer periods of time (2). Research suggests that female mosquitofish have a preference for males with similar body shapes to them, causing reproductive isolation and reinforcement between the ponds.