Genetic mechanisms and correlational selection structure trait variation in a coral snake mimic

The role of historical biogeography in shaping colour morph diversity in the common wall lizard

The maintenance of polymorphisms often depends on multiple selective forces, but less is known on the role of stochastic or historical processes in maintaining variation. The common wall lizard (Podarcis muralis) is a colour polymorphic species in which local colour morph frequencies are thought to be modulated by natural and sexual selection. Here, we used genome-wide single-nucleotide polymorphism data to investigate the relationships between morph composition and population biogeography at a regional scale, by comparing morph composition with patterns of genetic variation of 54 populations sampled across the Pyrenees. We found that genetic divergence was explained by geographic distance but not by environmental features. Differences in morph composition were associated with genetic and environmental differentiation, as well as differences in sex ratio. Thus, variation in colour morph frequencies could have arisen via historical events and/or differences in the permeability to gene flow, possibly shaped by the complex topography and environment. In agreement with this hypothesis, colour morph diversity was positively correlated with genetic diversity and rates of gene flow and inversely correlated with the likelihood of the occurrence of bottlenecks. Concurrently, we did not find conclusive evidence for selection in the two colour loci. As an illustration of these effects, we observed that populations with higher proportions of the rarer yellow and yellow-orange morphs had higher genetic diversity. Our results suggest that processes involving a decay in overall genetic diversity, such as reduced gene flow and/or bottleneck events have an important role in shaping population-specific morph composition via non-selective processes.

recolorize: An R package for flexible colour segmentation of biological images

Colour pattern variation provides biological information in fields ranging from disease ecology to speciation dynamics. Comparing colour pattern geometries across images requires colour segmentation, where pixels in an image are assigned to one of a set of colour classes shared by all images. Manual methods for colour segmentation are slow and subjective, while automated methods can struggle with high technical variation in aggregate image sets. We present recolorize, an R package toolbox for human-subjective colour segmentation with functions for batch-processing low-variation image sets and additional tools for handling images from diverse (high-variation) sources. The package also includes export options for a variety of formats and colour analysis packages. This paper illustrates recolorize for three example datasets, including high variation, batch processing and combining with reflectance spectra, and demonstrates the downstream use of methods that rely on this output.

Genetic mechanisms and correlational selection structure trait variation in a coral snake mimic

Covariation among traits shapes both phenotypic evolution and ecological interactions across space and time. However, rampant geographical variation in the strength and direction of such correlations can be particularly difficult to explain through generalized mechanisms. By integrating population genomics, surveys of natural history collections and spatially explicit analyses, we tested multiple drivers of trait correlations in a coral snake mimic that exhibits remarkable polymorphism in mimetic and non-mimetic colour traits. We found that although such traits co-occur extensively across space, correlations were best explained by a mixture of genetic architecture and correlational selection, rather than by any single mechanism. Our findings suggest that spatially complex trait distributions may be driven more by the simple interaction between multiple processes than by complex variation in one mechanism alone. These interactions are particularly important in mimicry systems, which frequently generate striking geographical variation and genetic correlations among colour pattern traits.