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Drosophila mojavensis comprises three geographic forms occurring in the United States, the Baja California peninsula, and mainland Mexico. Peninsular and mainland forms were selected for increased sexual isolation from each other, while controls were maintained with maximum outbreeding. Response to selection was highly asymmetrical in that isolation was very high between selected peninsular males and mainland females, but nonexistent between selected mainland males and peninsular females. The heightened isolation is primarily due to some change in the peninsular males.

We present results deriving from the inception of an investigation (involving 4,000 scored matings) into the development of discriminatory behavior in Drosophila paulistorum. We examined two complementary situations for their effects on mating behavior: physical isolation (seclusion) beginning at each of six developmental stages and exposure to heterosemispecifics during all preadult stages (mixed culturing). Preadult seclusion at certain stages decreased sexual isolation between three of four semispecific pairs. Seclusion effect, or lack thereof, varied with semispecies and with the stage at which seclusion was imposed: the earlier the stage at which seclusion was initiated, the more drastic the effect. In contrast, mixed culturing increased sexual isolation between the one pair of semispecies that was tested.