Experiments on Sexual Isolation in Drosophila

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Abstract
D. pseudoobscura and D. persimilis are morphologically highly similar sympatric populations which have hybridized only in the laboratory. In multiple choice expts. using virgin 7-day-old pseudoobscura [female][female] and 5 or 6-day old persimilis [female][female] (which mature more rapidly) it is demonstrated that sexual isolation is not complete but that conspecific matings are much more frequent than heterogamic matings. There were no apparent differences in mating behavior other than the greater activity of D. pseudoobscura: duration of copulation and pattern of behavior were approx. equal for both spp. Furthermore, "species recognition," if given a sensory explanation, does not adequately explain the data, though amputation of the organs of smell reduced the ratio of homogamy slightly. When multiple choice expts. were conducted under direct observation it was seen that [male][male] cf "display" equally to [female][female] of both spp. but the overwhelming majority of the heterogamic copulations remain incomplete and there must be some as yet undiscovered anatomical or physiological obstacle. The observer received the impression that it was the [female][female] who were mainly responsible for so many interspecific "incomplete copulations" (that is, copulations typical in every respect and including intromission of the [male] phallus and mounting, but terminated after 1-2 sec. rather than 4-11 min.).
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