More generally, alternative reproductive, anti-predator or resource-holding strategies among morphs drive the evolution of morph-specific optimal trait combinations (multiple fitness optima) that represent alternative solutions to the problem of optimizing fitness in the face of tradeoffs ( Whitlock et al., 1995 Sinervo, 2000 Roulin, 2004). Negative frequency–dependence is believed to maintain multiple morphs within a population ( Widemo, 1998). For example, in morphs of the lek-breeding ruff ( Philomachus pugnax), colour morphs exhibit alternative mating strategies that drive negative frequency–dependent selection. Often, the functional significance of a colour polymorphism is linked to the very mechanisms that maintain it. They may advertise social status ( Hover, 1985), mate compatibility ( Pryke & Griffith, 2009) or alternative life history or behavioural strategies ( Lepetz et al., 2009). Discrete colour signals can have a variety of functions. The complexity of these interactions makes it difficult to isolate mechanisms that maintain phenotypic variation, and as such, much of our current understanding comes from studies investigating the underlying function of discrete phenotypic polymorphisms that are easily tractable in the wild ( Brodie, 1992 Pryke & Griffith, 2006).Įlaborate colour signals are common in many species of vertebrates ( Seehausen & van Alphen, 1998 Roulin, 2004) and invertebrates ( Sandoval & Nosil, 2005 Svensson et al., 2005) and represent some of the most striking and well-documented examples of discrete phenotypic variation. Selection acts directly on phenotypic variation via individual interactions with the environment and with members of the same or different species (e.g. Our results suggest that multivariate selection pressures may favour alternative optimal morph-specific phenotypes in P. muralis.Ī central challenge in evolutionary biology is determining how phenotypic variation is preserved in natural populations despite persistent selection ( Bull, 1987 Houle, 1992 Rowe & Houle, 1996). We also find that morphs differ along multivariate phenotypic axes and experience different multivariate selection pressures. We show that ventral colour is a discrete trait and that morphs differ in body size, prevalence of infection by parasites and infection intensity. Here, we describe a ventral colour polymorphism in the wall lizard ( Podarcis muralis) and test the hypothesis that morphs differ along multivariate axes defined by trade-offs in morphological, physiological, and immunological traits. For example, trade-offs over resource allocation to morphological, physiological and behavioural traits can drive correlational selection for morph-specific phenotypic optima. Recent studies examining the morphological, physiological and behavioural differences among discrete colour morphotypes (morphs) have revealed several mechanisms that maintain discrete variation within populations, including frequency-dependence, density-dependence and correlational selection. A major goal in evolutionary biology is to determine how phenotypic variation arises and is maintained in natural populations.