The extent to which interspecific interference competition has contributed to character evolution is one of the most neglected problems in evolutionary biology. When formerly allopatric species come into secondary contact, aggressive interactions between the species can cause selection on traits that affect interspecific encounter rates (e.g. habitat preferences, activity schedules), competitor recognition (e.g. colouration, song), and fighting ability (e.g. weaponry, body size). We define agonistic character displacement (ACD) as the process of phenotypic evolution in a population caused by interference competition with one or more sympatric species and which results in shifts in traits that affect the rate, intensity or outcome of interspecific aggression. After clarifying the relationships between ACD and other evolutionary processes that may occur when species come into secondary contact, we develop an individual-based, quantitative genetic model to examine how traits involved in competitor recognition would be expected to evolve under different secondary contact scenarios. Our simulation results show that both divergence and convergence are possible outcomes, depending on the intensity of interspecific exploitative competition, the costs associated with mutual versus unilateral recognition, and the extent of phenotypic differences prior to secondary contact. We then devise a set of eight criteria for evaluating putative examples of ACD and review the empirical literature to assess the strength of existing evidence and to identify promising avenues for future research. Our literature search revealed 33 putative examples of ACD across insects, fishes, bats, birds, lizards, and amphibians (15 divergence examples; 18 convergence examples). Only one example satisfies all eight criteria for demonstrating ACD, but most case studies satisfy four or more criteria. The current state of the evidence for ACD is similar to the state of the evidence for ecological character displacement just 10 years ago. We conclude by offering suggestions for further theoretical and empirical research on ACD.

Life history traits in guppies (Poecilia reticulata) vary geographically along a predator assemblage gradient, and field experiments have indicated that the association may be causal; guppies introduced from high predation sites to low predation sites have evolved the phenotype associated with low predation in as few as seven generations. It has long been recognized, however, that low predation sites tend to have greater forest canopy cover than high predation sites. Stream differences in canopy cover could translate into stream differences in resource availability, another theoretically potent agent of selection on life history traits. Moreover, new computer simulations indicate that the high predation phenotype would outcompete the low predation phenotype under both mortality regimes. Thus, predation alone may not be sufficient to explain the observed life history patterns.Here we show that food availability for guppies decreases as forest canopy cover increases, among six low predation streams in the Northern Range of Trinidad. Streams with less canopy cover received more photosynthetically active light and contained a larger standing crop of algae (the primary food of guppies), as measured by algal pigments (chlorophylls and carotenoids) on both natural cobble and artificial tile substrates, but did not contain a greater biomass of guppies (per square meter of streambed). Consequently, algae availability for guppies (in micrograms of algal pigments per milligram of guppy) increased with decreasing canopy cover. The biomass of guppies and algae both decreased after a series of floods, with no net effect on algae availability. Field mark-recapture studies revealed that female and juvenile guppies grew faster, and that the asymptotic size of mature males was larger, in streams with less canopy cover. Canopy cover explained 84% of the variation among streams in algae availability which, in turn, explained 93% of the variation in guppy growth rates. Laboratory ''common garden'' experiments indicated that the stream differences in growth and adult male size in the field were largely environmental (nongenetic). These results strongly suggest that stream differences in canopy cover result in consistent stream differences in food availability, independent of predation.Our preliminary data indicate that some life history traits (offspring size and litter size) vary genetically along the canopy cover gradient, among low predation streams, in the same direction as along the predation gradient. Another recent study shows that food availability is higher at high predation sites than at low predation sites, partly as an indirect effect of predators reducing guppy densities. Further research is required to disentangle the direct effects of predation from those of resource availability in the evolution of life histories.

Carotenoids produce most of the brilliant orange and yellow colours seen in animals, but animals cannot synthesize these pigments and must rely on dietary sources. The idea that carotenoids make good signals because they are a scarce limiting resource was proposed two decades ago and has become the leading hypothesis for the role of carotenoids in animal communication. To our knowledge, until now, however, there has been no direct evidence that carotenoids are a limiting resource in nature. We showed that carotenoid availability in the wild limits the expression of sexual coloration in guppies (Poecilia reticulata), a species in which females prefer males with brighter orange carotenoid-containing spots. Further, the degree of carotenoid limitation varies geographically along a replicated environmental gradient (rainforest canopy cover), which opens new avenues for testing signal evolution theory.

In most animals, the origins of mating preferences are not clear. The 'sensory-bias' hypothesis proposes that biases in female sensory or neural systems are important in triggering sexual selection and in determining which male traits will become elaborated into sexual ornaments. Subsequently, other mechanisms can evolve for discriminating between high-and low-quality mates. Female guppies (Poecilia reticulata) generally show a preference for males with larger, more chromatic orange spots. It has been proposed that this preference originated because it enabled females to obtain high-quality mates. We present evidence for an alternative hypothesis, that the origin of the preference is a pleiotropic effect of a sensory bias for the colour orange, which might have arisen in the context of food detection. In field and laboratory experiments, adult guppies of both sexes were more responsive to orange-coloured objects than to objects of other colours, even outside a mating context. Across populations, variation in attraction to orange objects explained 94% of the inter-population variation in female mate preference for orange coloration on males. This is one of the first studies to show both an association between a potential trigger of a mate-choice preference and a sexually selected trait, and also that an innate attraction to a coloured inanimate object explains almost all of the observed variation in female mate choice. These results support the 'sensorybias' hypothesis for the evolution of mating preferences.

Colour patches are complex traits, the components of which may evolve independently through a variety of mechanisms. Although usually treated as simple, two-dimensional characters and classified as either structural or pigmentary, in reality colour patches are complicated, three-dimensional structures that often contain multiple pigment types and structural features. The basic dermal chromatophore unit of fishes, reptiles and amphibians consists of three contiguous cell layers. Xanthophores and erythrophores in the outermost layer contain carotenoid and pteridine pigments that absorb short-wave light ; iridophores in the middle layer contain crystalline platelets that reflect light back through the xanthophores ; and melanophores in the basal layer contain melanins that absorb light across the spectrum. Changes in any one component of a chromatophore unit can drastically alter the reflectance spectrum produced, and for any given adaptive outcome (e.g. an increase in visibility), there may be multiple biochemical or cellular routes that evolution could take, allowing for divergent responses by different populations or species to similar selection regimes. All of the mechanisms of signal evolution that previously have been applied to single ornaments (including whole colour patches) could potentially be applied to the individual components of colour patches. To reach a complete understanding of colour patch evolution, however, it may be necessary to take an explicitly multi-trait approach. Here, we review multiple trait evolution theory and the basic mechanisms of colour production in fishes, reptiles and amphibians, and use a combination of computer simulations and empirical examples to show how multiple trait evolution theory can be applied to the components of single colour patches. This integrative perspective on animal colouration opens up a host of new questions and hypotheses. We offer specific, testable functional hypotheses for the most common pigmentary (carotenoid, pteridine and melanin) and structural components of vertebrate colour patches.