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,
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 observe 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.