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RESEARCH PRODUCT

POSTMATING ISOLATION ANALYSIS IN FOUNDER-FLUSH EXPERIMENTAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA.

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In this paper, we report a detailed analysis intended to detect postmating barriers in experimental populations of Drosophila pseudoobscura obtained through nine founderflush cycles. The number of offspring produced was determined in three consecutive generations of hybridization. It is found that the evolution of premating barriers, as shown by two of these populations, is not necessarily accompanied by the evolution of postmating ones. Under the founder-flush model of speciation proposed by Carson (1971), the first step in the speciation process is usually thought to be an incidental genetic change through founder effect affecting mating behavior so that sexual premating isolation evolves. Postmating barriers are unnecessary under this theory, although they may evolve incidentally afterwards. This order of events is exactly opposite to that predicted by the reinforcement speciation model (Dobzhansky 1951; Ayala 1991), in which allopatric populations develop postmating barriers (cross-sterility, inviability of hybrids, or hybrid breakdown) and, after secondary contact, premating barriers may evolve adaptively. Founder-flush experiments performed to date have concentrated on premating barriers among experimentally bottlenecked populations, mainly searching for sexual isolation with multiple choice mating tests (Powell 1989; Ringo et al. 1985; Meffert and Bryant 1991; Galiana et al. 1993). With respect to postmating barriers, Powell (1989) did not detect sterility nor morphologic anomalies of genitalia in his hybrids, and Ringo et al. (1985) found a slight but progressive reduction in average fertility over the six studied founderflush-crash cycles. Here we report an analysis of fertility with four populations of D. pseudoobscura from another founderflush experiment (Galiana et al. 1989, 1993). This study was performed for a small number of populations and only during one founder-flush cycle. We concentrated our effort on a detailed analysis of fertility, over three consecutive generations of hybridization.