Female animals who prefer healthy and fit partners are having more and fitter descendants. A gene that brings forth such mating preferences is going to spread quickly. This gene can even forward the split-up of one species into two.
Schindler, S., Breidbach, O., & Jost, J. (2013). Preferring the fittest mates: An analytically tractable model Journal of Theoretical Biology, 317, 30-38 DOI: 10.1016/j.jtbi.2012.09.018