As effective population size (Ne) decreases, genetic factors may become relatively important to a population's or species' persistence. Conservation biologists should be aware of anything that can potentially cause a sudden reduction in Ne. I used simple models to illustrate how certain types of female mating preferences combined with certain types of male traits may lead to a sudden and substantial decrease in Ne. Specifically, if and when there is a sudden ‘downward’ shift in the expression of condition-dependent male traits, females using fixed-threshold mate choice criteria might find fewer acceptable males. While mechanisms of female choice remain elusive, a variety of sexually selected traits may be condition dependent. Because the expression of condition-dependent traits is likely to be impacted by natural or human-induced environmental changes, behavioral and conservation biologists should pay special attention to them around the mating season. Armed with knowledge of condition-dependent male traits, it may be possible to minimize the impact on condition-dependent traits while planning translocations or reintroductions.