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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]

Behavioural phenotypes are invariably plastic to some degree and are among the most labile of phenotypes. Some are acquired over the course of development in a particular environment (developmental plasticity), but most are elicited by an environmental trigger and are expressed only briefly, but often repeatedly, in the life of an organism (activational plasticity). Thus, individuals can possess the ability to perform a behaviour, but in the absence of the appropriate environmental stimulus, can fail to do so over the course of a lifetime. Rarely is the evolution of behavioural phenotypes explored in the larger context of the evolution of phenotypically plastic traits. Here I argue that the evolution of behavioural phenotypes, regardless of the nature of the plasticity expressed, can be examined in the same way as the evolution of other plastic traits. I first provide a conceptual review of the factors that can influence the evolution of plastic phenotypes and of the ways in which behavioural plasticity can influence the evolution of other aspects of phenotype. Many of the most compelling questions involve contrasting ancestral patterns of plasticity with those in derived populations or species, a particularly challenging problem in the case of phenotypic plasticity. I therefore provide an overview of the ways in which the influence of plasticity upon evolution can be addressed and then provide a review of examples from the literature that offer initial insights into the role of behavioural plasticity in evolution. The questions are exciting, the data limited, and, as I argue in closing, we need creative insights into the ways in which behavioural plasticity has evolved and has in turn influenced the evolution of other aspects of phenotype. [ABSTRACT FROM AUTHOR]

Describes field observations of a classical example of an alterntive male mating tactic: sneaking by male three-spined sticklebacks, Glasterosteus aculeatus, the context in which it occurred and evaluates possible causes of variation in sneaking between and within populations. Methods; Results.

At the onset of adaptive differentiation caused by colonization of new environments, patterns of ancestral plasticity can influence trait expression, and thus determine which phenotypes are exposed to selection. This hypothesis can be evaluated in the postglacial adaptive radiation of threespine stickleback, Gasterosteus aculeatus, in northwestern North America. The very large oceanic populations that gave rise to the freshwater radiation are unlikely to have changed significantly since colonization was initiated, so the pattern of plasticity in oceanic fish can be used to infer the ancestral pattern. We demonstrate that courtship behaviour in oceanic sticklebacks is plastic and suggest that its expression is mediated by large cannibalistic foraging groups that attack and consume young in nests defended by males. We then demonstrate that, under laboratory conditions, males from two benthic (bottom-feeding) populations, where group cannibalism is common, zigzag less than males from a limnetic (plankton-feeding) population in which such groups do not form. The difference in courtship behaviour of these ecotypes parallels that seen in the field and the pattern of plasticity observed in oceanic fish. Ecotypic differences were also retained in laboratory-reared fish from one limnetic and one benthic population. Biting was a more common component of courtship in the laboratory than in the field, and males from freshwater populations bit more than those from the oceanic population, a difference that disappeared in laboratory-reared benthic and limnetic males. Our results suggest that ancestral plasticity may have influenced the subsequent evolution of some elements of courtship behaviour in the stickleback radiation. [Copyright &y& Elsevier]

Changing environments, whether through natural or anthropogenic causes, can lead to the loss of some selective pressures (‘relaxed selection’) and possibly even the reinstatement of selective agents not encountered for many generations (‘reversed selection’). We examined the outcome of relaxed and reversed selection in the adaptive radiation of the threespine stickleback fish, Gasterostues aculeatus L., in which isolated populations encounter a variety of predation regimes. Oceanic sticklebacks, which represent the ancestral founders of the freshwater radiation, encounter many piscivorous fish. Derived, freshwater populations, on the other hand, vary with respect to the presence of predators. Some populations encounter native salmonids, whereas others have not experienced predation by large fish in thousands of generations (relax-selected populations). Some relax-selected populations have had sport fish, including rainbow trout, Oncorhynchus mykiss , introduced within the past several decades (reverse-selected). We examined the behavioural responses of sticklebacks from three populations of each type to simulated attacks by trout and birds to determine whether relaxed and reversed selection has led to divergence in behaviour, and whether this divergence was predator specific. Fish from trout-free populations showed weak responses to trout, as predicted, but these responses were similar to those of oceanic (ancestral) populations. Fish from populations that co-occur with trout, whether native or introduced, showed elevated antipredator responses, indicating that in freshwater, trout predation selects for enhanced antipredator responses, which can evolve extremely rapidly. Comparison of laboratory-reared and wild-caught individuals suggests a combination of learned and genetic components to this variation. Responses to a model bird flyover were weakly linked to predation environment, indicating that the loss of predation by trout may partially influence the evolution of responses to birds. Our results reject the hypothesis that the consistent presence of predatory birds has been sufficient to maintain responses to piscivorous fish under periods of relaxed selection. [ABSTRACT FROM AUTHOR]