Reasons to be different: do conspicuous polymorphisms in invertebrates persist because rare forms are fitter?

Many invertebrate species show conspicuous colour polymorphisms, the study of which has provided us with important insights in evolutionary biology. The potential importance of frequency‐dependent selection in the maintenance of polymorphisms was identified by theoretical studies more than 50 years ago, and since then, the topic has received considerable attention from those seeking to explain observed diversity in natural populations. Here, we consider the different ecological interactions that have been shown to lead to negative frequency‐dependent selection in invertebrate populations in the wild, and assess the likely relative importance of this mechanism in comparison with alternatives that may promote genetic and phenotypic diversity. The literature shows that frequency dependence can result from a wide array of ecological interactions, in particular, those involving mate choice, sexual conflict and predation. However, even though negative frequency‐dependent selection is the most common explanation for the occurrence of conspicuous polymorphisms in invertebrates, conclusive evidence of its importance in natural populations is largely absent. A particular problem is that in most studies, it is the only explanation considered. In the most comprehensively studied systems, it has been shown that multiple mechanisms (both selective and neutral) operate to maintain observed phenotypic variation, and that negative frequency‐dependent selection is not the most important of these. Thus, as yet at least, we do not have strong grounds for believing that negative frequency‐dependent selection is a major diversifying force in invertebrate morphology. However, without more comprehensive studies in a wider range of ecological contexts, we are equally unable to dismiss it as weak and/or irrelevant.