Drosophila simulans is a species of fly closely related to D. melanogaster and which belongs to the same melanogaster species subgroup. Its closest relatives are D. mauritiana and D. sechellia. This species was discovered by the fly geneticist Alfred Sturtevant in 1919, when he noticed that the flies used in Thomas Hunt Morgan's laboratory at the Columbia University were actually two distinct species: D. melanogaster and D. simulans. Males differ in the external genitalia, while trained observers can separate females using colour characteristics. D. melanogaster females crossed to D. simulans males produce sterile F1 females and no F1 males. The reciprocal cross produces sterile F1 males and no female progeny.
D. simulans was found later to be closely related to two island endemics, Drosophila sechellia and Drosophila mauritiana. D. simulans will mate with these sister species to form fertile females and sterile males, a fact that has made D. simulans an important model organism for research into speciation.
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