![]() ![]() This hypothesis remains untested in the macroevolutionary literature, but we predict that size-constrained signals should exhibit both (i) limited phenotypic space for exaggeration and (ii) reduced standing variation. This may also influence other aspects of signal design, such as standing variation (the degree to which phenotypes vary within a species). In other words, the primary consequence of constraint is a limit to the range of viable signal phenotypes. These effects are also apparent on a macroevolutionary basis, where morphological constraints restrict the potential for display divergence. In these examples, small crabs cannot hold up a large claw to wave, and smaller spiders produce less intense vibrations on their web thus, signal exaggeration (claw size or vibration amplitude) is constrained by body size. Indeed, individual size differences limit the exaggeration of displays as diverse as crab claw-waves and spider seismic signals. One of the main constraints on signal design is morphology. The interactions between multiple evolutionary drivers should therefore affect the macroevolutionary patterning of complex displays, but this topic is rarely addressed. However, despite phenotypic and functional differences in each module, the full display still relies on the totality of all components rather than any one in isolation. This means that complex displays are modular: different signal components, or ‘modules’, each serve different functions and undergo their own evolutionary trajectory. Accordingly, complexity provides the phenotypic foundation for sexual selection to continue past the effects of constraint by allowing multiple signals to independently undergo modification. Prior studies suggest that the solution may lie in the evolution of complex displays, which are constructed from more than one component signal or element. ![]() In the current study, we explore this issue by testing how morphological constraints on a signal's production influence sexual selection for its design. ![]() Although studies through the latter lens reveal that both constraint and selection profoundly influence display elaboration, the inherently complex nature of signal design suggests that these fundamental processes may work in unexpected ways. Work on a proximate scale shows that sexual selection operates differentially based on the diverse constraints that influence signals, and thus it is an ongoing challenge to understand how this microevolutionary process informs phenotypic patterning at the macroevolutionary scale. Here, constraints define boundaries that limit phenotypic exaggeration and standing variation, while selection elaborates displays by exaggerating existing components or by favouring the emergence of new display traits altogether. Most prominent is the conflict between constraints and selection. Modular displays therefore provide the basis for selection to find novel routes to phenotypic elaboration after previous ones are closed.Ĭomplex animal displays diversify in response to a tug-of-war between multiple evolutionary pressures. ![]() This suggests that when morphology limits the exaggeration of one component, sexual selection instead exaggerates the unconstrained trait. We also uncover a positive relationship between sexual size dimorphism and the unconstrained trait (length), but with no effect on speed. As a result of this biomechanical constraint, there is less standing variation in speed than length. We report that species body size limits drum speed, but not drum length. These territorial displays are produced when an individual rapidly hits its bill on a hard surface, and drums vary across species in the number of beats included (length) and the rate of drumbeat production (speed). This is seldom studied on a phylogenetic scale, but we address the issue herein by studying macroevolutionary patterning of woodpecker drum displays. How do interactions between selection and constraint shape display evolution? One possibility is that sexual selection continues exaggeration under constraint by operating differentially on each signal component in complex, modular displays. Sexual selection drives elaboration in animal displays used for competition and courtship, but this process is opposed by morphological constraints on signal design. ![]()
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