Drosophila simulans

Drosophila simulans is a species of fruit fly closely related to "D. melanogaster" 
It 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.