Origins and significance of preference for rare/novel male morphs: Trinidadian guppies as a case study

Recent evidence suggests that negative frequency-dependent selection may be a pervasive explanation for the high genetic variation observed in ecologically important traits. Female mating preference for rare/novel male phenotypes is one process that can generate frequency-dependent selection and has been well documented in some highly polymorphic systems. However, two major gaps in the literature limit the current ability to predict the taxonomic prevalence of this mating preference. First, tests of this preference historically suffered from pitfalls that limited interpretability. We discuss best practices for designing convincing tests of preference for rare/novel morphs. We argue that the literature on guppies provides robust evidence of the preference in the laboratory and field, but published tests remain limited in other species and on traits other than morphological ones. Second, an evolutionary explanation for the preference remains elusive, making it difficult to predict the conditions in which the preference should arise and be maintained. We outline several hypotheses proposed to explain the preference and synthesize evidence supporting or contradicting them. For both gaps, we suggest key, tractable avenues for future work. We hope to stimulate research that will elucidate the evolutionary cause(s) of preference for rare/novel morphs and its potentially widespread role in maintaining genetic variation in natural populations. This article is part of the Theme Issue 'Exploring negative frequency dependent selection across levels: from genetics to ecology and back again'.