Avian predation intensity as a driver of clinal variation in colour morph frequency

Abstract

Phenotypic variation provides the framework for natural selection to work upon, enabling adaptive evolution. One of the most discernible manifestations of phenotypic variability is colour variation. When this variation is discrete, genetically based colour pattern morphs occur simultaneously within a population. Why and how colour polymorphisms are maintained is an evolutionary puzzle. Several evolutionary drivers have been hypothesized as influencing clinal patterns of morph frequency, with spatial variation in climate and predation being considered especially important. Despite this, no study has examined both of their roles simultaneously. The aims of this study were to: (a) examine the covariation of physiology, environmental variables and colouration at a local scale; and (b) determine if these factors and their interplay explain broad clinal variation in morph frequency. We used the lizard Liopholis whitii as a model system, as this species displays a discrete, heritable polymorphism for colour pattern (plain-backed, patterned morphs) whose morph frequency varies latitudinally. We measured reflectance, field activity temperatures and microhabitat structure to test for differences in crypsis, thermal biology and microhabitat selection of patterned and plain-backed morphs within a single population where colour morphs occur sympatrically. We then used data from the literature to perform a broad-scale analysis to identify whether these factors also explained the latitudinal variation of morph frequency in this species. At the local scale, plain-backed morphs were found to be less cryptic than patterned morphs while no other differences were detected in terms of thermal biology, dorsal reflectance and microhabitat use. At a broader scale, predation was the most influential factor mediating morph frequency across latitudes. However, the observed pattern of morph frequency is opposite to what the modelling results suggest in that the incidence of the least cryptic morph is highest where predation pressure is most severe. Clinal variation in the level of background matching between morphs or the potential reproductive advantage by the plain-backed morph may, instead, be driving the observed morph frequency. Together, these results provide key insights into the evolution of local adaptation as well as the ecological forces involved in driving the dynamics of colour polymorphism.

Examining the link between relaxed predation and bird coloration on islands

“…To estimate predation pressure, we collated data on the average specific richness of sympatric avian predators across the range of each species [23][24][25][26][27][28][29] and computed predator-prey mass allometry relationships to exclude some predators based on size mismatch with potential preys [23,30] (electronic supplementary material, appendix S3). Predator densities or species-specific estimates of raptor-driven mortality would provide additional resolution but are not available for such a large-scale study.…”

Using crowdsourced photographic records to explore geographical variation in colour polymorphism

“…Although our analytical framework is correlative in nature, our findings provide compelling insights into the climatic selective processes that might have led to such geographical variation in colour polymorphism, and the plausible ecological functions that ensue as a result of each morph. Colouration often has direct effects on thermoregulation and camouflage, both of which influence survival and hence evolutionary fitness (Cheng et al, 2018; Clusella‐Trullas et al, 2008; Matthews et al, 2018; Olsson et al, 2013; Xing et al, 2016). Herein, we discuss the potential for climatic selection on thermoregulation and camouflage to generate the observed geographical distribution of colour morphs in V. varius .…”

Balancing selection and drift in a polymorphic salamander metapopulation

“…Finally, although balancing selection could stabilize morph frequencies locally, the identity of the possible stabilizing selection on equilibrium morph frequency remains unknown. Given widespread evidence for clinal variation in polymorphism frequencies in other systems [91][92][93], regional differences in selection seem likely.…”