Evolution of Increased "Female Sex Ratio" in the Blowfly (Phaenicia sericata) Under Laboratory Competition with the Housefly (Musca domestica)

Abstract

Musca domestica and Pbaenicia sericata were kept together in competition in a 16-cell population cage. Severe larval and adult competition for food and space occurred. Neither species is cannibalistic nor predaceous on the other in any life history stage, and both have polygamous males. The adults were censused weekly, and the sex ratio of a sample determined periodically. Musca went to extinction between the sixty-fourth and sixtyfifth censuses. The cage was discontinued at the sixty-ninth census. Significant preponderances of Pbaeaicia female adults were observed in the population cage after one-half year. This preponderance of females was not temporary and merely existent during the pressure of competition, since the immediate descendants of the last generation phaenicia cage population reared in uncrowded conditions also bad a high female sex ratio which was not significantly different from that observed in the Phaenicia population in the cage except during one census. Furthermore, the sex ratio at emergence of wild stock Phaenicia from larvae reared under intraspecific competition during one generation did not change from its normal condition. This ratio was significantly different from Phaenicia descendants of the cage population reared in the absence of competition. It was concluded that a higher female sex ratio at emergence evolved in the Phaenicia population in the population cage. This would have been advantageous under the severe interspecific competitive conditions existing in the population cage system.