Preference divergence is thought to contribute to reproductive isolation. Ecology can alter the way selection acts on female preferences, making them most likely to diverge when ecological conditions vary among populations. We present a novel mechanism for ecologically dependent sexual selection, termed ‘the ecological stage’ to highlight its ecological dependence. Our hypothesized mechanism emphasizes that males and females interact over mating in a specific ecological context, and different ecological conditions change the costs and benefits of mating interactions, selecting for different preferences in distinct environments and different male traits, especially when traits are condition dependent. We test key predictions of this mechanism in a sympatric threespine stickleback species pair. We used a maternal half-sib split-clutch design for both species, mating females to attractive and unattractive males, and raising progeny on alternate diets that mimic the specialized diets of the species in nature. We estimated the benefits of mate choice for an indicator trait (male nuptial color) by measuring many fitness components across the lifetimes of both sons and daughters from these crosses. We analyzed fitness data using a combination of aster and mixed models. We found that many benefits of mating with high color males depended on both species and diet. These results support the ecological stage hypothesis for stickleback. Finally, we discuss the potential role of this mechanism for other taxa and highlight its ability to enhance reproductive isolation as speciation proceeds, thus facilitating the evolution of strong reproductive isolation.

We provide all data and code necessary to recreate the results presented in the manuscript. ReadMe files are also provided for each aster model in the manuscript (survival, fecundity, mating).