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The fitness effects of extreme genetic change by selection were studied in large populations subjected to prolonged, intense selection. Two replicate populations of Drosophila melanogaster, with estimated effective sizes 500 [less than or equal to] [N.sub.e] [less than or equal to] 1000, were selected for increased performance in a wind tunnel, selecting on average the fastest 4.5% of flies. The mean apparent flying speed of both lines increased from [approximately]2 to 170 cm/sec and continued to respond at diminishing rates, without reaching a plateau, for 100 generations. Competitive fitness tests in generations 50 and 85 showed minimal or no fitness loss in selected lines compared to controls. Sublines relaxed in generations 65 and 85 showed minimal or no regression in apparent flying speed. Hybrid lines, from a cross of selected x control lines in generation 75, responded to reselection saltationally, showing that the chromosomes of the selected lines had been assembled from alleles at many loci, from many different chromosomes in the base population. Thus, major genetic change was achieved, but without the costs usually associated with strong directional selection. Large population size has been interpreted, in opposing models, as either a brake or an accelerator in its effects on long-term change by selection. These results favor the second model, and challenge the concept of rugged fitness surfaces underlying the first model.