Experimental evidence for testis size evolution via sperm competition

Sperm competition theory predicts increased spermatogenic investment with i ncreased sperm competition risk when competition is numerical. There is amp le correlational evidence for this relationship in a wide range of taxa. Ho wever, as with all correlations, this does not establish cause and effect. Nevertheless, there are no published experimental studies of the evolutiona ry influence of sperm competition on testis size. We report here on evoluti onary responses of testis size to variation in sperm competition intensity in the yellow dung fly. Experimental flies were divided across two treatmen ts, polyandrous or monogamous, with four replicates of each. There was a ra pid evolutionary response in testis size resulting from selection via sperm competition, with larger testes found when sperm competition intensity was greatest. These results provide direct experimental evidence of evolutiona ry change consistent with macro-evolutionary patterns found across a wide r ange of taxa.