Population Genomic Evidence Reveals Subtle Patterns of Differentiation in the Trophically Polymorphic Cuatro Ciénegas Cichlid, Herichthys minckleyi

Variability and the primacy of the genotype

Cave animals and species flocks exhibit common evolutionary principles. In caves, all traits dependent on the information derived from light lose their biological function. Mutations destructive for such traits, but neutral for the organism as a whole, can persist and accumulate until a trait has vanished. Adaptive radiations start in ecosystems containing open niches. Here, selection on niche-specific traits, such as the viscerocranium in fish, is relaxed owing to the absence of competing species, and viscerocranial variability arises. It is transitorily high in recent and phylogenetically younger flocks, providing new phenotypes. It lessens and is completely lost after directional selection promotes the fixation of phenotypes that are best adapted. In cave animals and species flocks, single traits manifest phenotypic variability owing to relaxed selection. Like the eye in cave species, the viscerocranium can be classified a module, the development of which is encoded in gene regulatory networks. Mutations in these genes can result in new phenotypes. Regarding functionality, these mutations might be destructive and eliminated by selection, neutral and thus persisting, or beneficial and promoted to fixation by directional selection. Given the ancient heritage of teleostean fish, these gene regulatory networks might be prone to mutations at the same loci or to developmental reactions resulting in similar phenotypes in closely related or taxonomically and geographically distant species.