Background and AimsAlthough evolution of sexual polymorphism has been traditionally analysed using discrete characters, most of these polymorphisms are continuous. This is the case of heterostyly. Heterostyly is a floral polymorphism successfully used as a model to study the evolution of the sexual systems in plants. It involves the reciprocal positioning of anthers and stigmas in flowers of different plants within the same population. Studies of the functioning of heterostyly require the quantification of the degree of reciprocity between morphs of heterostylous species. Some reciprocity indices have been proposed previously, but they show significant limitations that need to be dealt with. This paper analyses these existing indices, and proposes a new index that aims to avoid their main problems (e.g. takes into account population variability and offers a single value per population).
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