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Polymorphic species, in which multiple variants coexist within a population, are often used as model systems in evolutionary biology. Recent research has been dominated by the hypothesis that polymorphism can be a precursor to speciation. To date, the majority of research regarding polymorphism and speciation has focused on whether polymorphism is maintained within a population or whether morphs within populations may diverge to form separate species (sympatric speciation); however, the geographical context of speciation in polymorphic systems is likely to be both diverse and complex. In this review, we draw attention to the geographic variation in morph composition and frequencies that characterises many, if not most polymorphic species. Recent theoretical and empirical developments suggest that such variation in the number, type and frequency of morphs present among populations can increase …