SEXUAL SELECTION IN A NATURAL POPULATION OF DROSOPHILA MELANOGASTER

Drosophila melanogaster is among the best known organisms for laboratory studies about genetics and evolution. In laboratory conditions, many instances of selection have been observed (for reviews see Hedrick and Murray [1983], Wright [1977]). Yet in spite of several well known clines that are thought to result from selection (e.g., Stalker, 1980; Oakeshott et al., 1981, 1982), no instances of selection have been observed directly for free-living populations of this species, largely because its ecology has been so poorly known. Selection in D. melanogaster is of special interest because of the opportunity to relate knowledge of its molecular biology to its evolution. Recently acquired understanding about the natural history of this species permitted us to observe whether sexual selection, which occurs for wing length of males in laboratory situations (Ewing, 1961, 1964; Partridge and Farquhar, 1983; Hoffman, 1986), is also found in free-living populations. Wing length of males that were found copulating on natural leks were compared to random samples of males that were present there but not copulating. Those males that were successful at obtaining mates had larger wings than males from a random sample. No difference in size was observed between males on major and minor leks, nor between males located on the leks and those found nearby, but off of the lek itself. The study was conducted during August 1982 and 1985 in a small orchard and associated brandy-making hut, or pecera, near a small rural village, Sremska Kamenica, in Yugoslavia. Many of the fruits raised there are used by the Drosophila. A comprehensive account of their ecology and mating behavior there will be published elsewhere (Taylor and Kekic, unpubl.). Only a brief account is given here. During August, the flies oviposit primarily on broken grapes in the surrounding vinyard and the larvae mature there. The adults feed not on the grapes, but on fermenting fruit (peaches, apricots, and plums), which is used as mash for making brandy and is contained in large barrels in or adjacent to the pecera. Mating occurs almost exclusively on the inside walls of these barrels; surveys conducted during the period of this study, made by surveying all of the flies in the pecera at hourly intervals, showed that, of 87 matings, 84 occurred on the walls and rim of the barrels, three occurred on the mash surface, and none occurred elsewhere in the pecera. Much more mating occurs on some containers than on others. Mating does occur outside the pecera, e.g., on grapes in the vinyard, but is rare there. A strong diurnal rhythm governs the flies' sexual activity, with nearly all mating activity occurring between 5:00 and 8:00 A.M. The sex ratio varies throughout the day and from place to place, but averages about 4:3 (females: males) on the surface of the food and 1:3.5 (females: males) on the wall of the containers. During the period when most mating occurs, males on the food surface and container wall move about and court vigorously in a frenzied manner. At the same time, males elsewhere, e.g., on the pecera wall, are nearly motionless and display virtually no courtship activity. It is not necessary that the females go to the sides of the container where mating occurs. They are free to fly directly onto the surface and then fly off (and they frequently do). Often, however, they also visit the sides where mating occurs. While the mating area is immediately adjacent to the food, it is not on the feeding area itself, and so the flies are in probable accord with the standard definition of a lek as "assemblies of adult males which females visit solely for the purpose of copulation" (Bradbury and Gibson, 1983). They are certainly in accord with the less restrictive notion of a "resource-based lek" (Alexander, 1975). For the 1982 collection, we collected at hourly intervals throughout two days and obtained 33 copulating pairs from the containers of mash associated with the pecera. Noncopulating males were sampled randomly from the leks by disturbing the containers and capturing flies with a net as they departed. Wing length of the males was measured along the fifth longitudinal vein, from the anal crossvein to the posterior crossvein. For 21 males captured in copulo, the mean wing length (? SE) was 73.4 ? 0.9 x 10-2 mm. A random sample of a like number of noncopulating males had a mean wing length of 72.4 ? 1.3 x 10-2 mm. While the difference is in the expected direction, it is not statistically significant (t = 0.67, 40 d.f, 0.20 < P < 0.30). The measurements were repeated during 1985, this time using the length of the third longitudinal vein from the anterior crossvein to the distal edge. For 55 males captured in copulo during a two-day period, the mean wing length (? SE) was 141.8 ? 1.2 x 10-2 mm, and for 55 males from a random sweep at the same times the mean was 137.2 ? 1.3 x 10-2 mm. This difference is statistically significant (t = 2.62; 108 d.f, P < 0.005). Again, those males that were more successful at obtaining mates were larger than random, leading to the conclusion that there was sexual selection for longer wing length, with a selection intensity (Falconer, 1981) of approximately 0.48. The distributions of wing lengths are shown in Figure 1. Their variances were not sig-