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Behavioural phenotypes are notable for their plasticity, in that individual behaviour patterns can be expressed ephemerally in response to an appropriate stimulus and then disappear. In the absence of an appropriate stimulus, behavioural phenotypes can remain unexpressed over many generations, yet the capacity to perform the behaviour can be retained. Here we discuss potential evolutionary influences of unexpressed behavioural phenotypes using two examples, one from the post-glacial adaptive radiation of the threespine stickleback fish, Gasterosteus aculeatus, and one from a far more ancient radiation, that of Pheidole ants. These radiations demonstrate that unexpressed phenotypes can persist for thousands or more generations in a condition that permits re-expression when an appropriate stimulus appears in the environment. We describe possible explanations for persistence of unexpressed phenotypes and demonstrate that in the absence of an appropriate phylogeny, re-expressed traits could be interpreted as true novelties in the group. We then discuss the way in which loss of expression can lead to parallelism in adaptive radiations — a mechanism that is rarely considered. We conclude by examining the way in which reappearance of unexpressed ancestral traits has the potential to facilitate population persistence if ancestral environments reappear and discuss the reasons that the evolutionary implications of behavioural plasticity are not better studied. • Plastic behavioural phenotypes can remain unexpressed for generations yet reappear. • Re-emergent behavioural phenotypes can appear novel when their origin is ancient. • Loss of expression of plastic behaviour can lead to parallelism in adaptive radiations. • Re-emergent phenotypes may rescue populations when ancestral environments reappear. • Evolutionary implications of plasticity are not trivial but remain poorly studied. [ABSTRACT FROM AUTHOR]