LAUSR

5 The operation of sexual selection depends on ecological conditions. Thus, changes in 6 environmental conditions because of human activities can alter the strength and direction of 7 sexual selection, with implications for evolutionary trajectories and the viability of populations. 8 We show that aquatic algal blooms can relax the operation of sexual selection by influencing 9 which males are available to attract females. This is by influencing the ability of males to maintain 10 a resource needed in mate attraction. When we exposed two competing threespine stickleback 11 males (Gasterosteus aculeatus), whose attractiveness to females was known, to either clear or 12 algal-turbid water, nest abandonment was more common in clear water, and it was usually the 13 unattractive male that abandoned his nest. In turbid water, on the other hand, nest 14 abandonments were less common and when they occurred, the probability increased that the 15 attractive male abandoned his nest, and that the unattractive male subsequently occupied it. This 16 change in the composition of nesting males increased the mating success of unattractive males. 17 Thus, our results reveal a new mechanism through which habitat deterioration can influence 18 sexual selection, by altering success in the competition for a resource critical in mate attraction, a 19 territory with a nest in this case. This could be a common mechanism, considering the prevalence 20 of resource competition in mate choice systems. On a broader level, our results emphasise the 21 importance of considering the impact of environmental changes on the outcome of resource 22 competition when investigating the influence that environmental disturbances have on the 23 operation of sexual selection and thereby on evolutionary processes and population dynamics. 24