Despite tradition, like the influence of the genotype-phenotype

Despite the great achievements of the Modern Evolutionary Synthesis theoretical efforts, in recent times a growing body of opinion has suggested that the study of heritable variation (one of the pillars of the evolutionary theory), its origin and its structure, has been substantially neglected citep{ pigliucci2007we}. For nearly 150 years after the publication of the Origin of species by Charles Darwin citep{darwin1859origin}, evolutionary biology focused primarily on the role of selection, chance and inheritance in the adaptation process and on the dynamics of molecular and morphological evolution citep{gould2002structure}. Actually, not only evolution influences variation, but the latter can also play an “instructive” role, rather than merely a “permissive” one, in determining evolutionary outcomes citep{Fusco:2012aa, gould2002structure}. This heterogeneous collection of studies are grouped, among other, under the umbrella term of  “extended evolutionary synthesis” citep{huxley2010evolution}, which includes for instance the interdisciplinary field of studies known as evolutionary developmental biology (or, evo-devo). General concepts deriving from the evo-devo tradition, like the influence of the genotype-phenotype map structure on evolution, also apply to living systems. In fact while evo-devo claims that development can bias the production of phenotypic variation, it is not true that the structure of variation, its instructive role, must come from development exclusively. Indeed, development is only a segment of an organisms’ life cycle citep{minelli2010developmental} and there are biological processes other than development that can be source of anisotropic phenotypic variation. These are for example standard mutation and recombination through the constrains imposed by standing genetic architecture (e.g.,cite{hansen2006evolution}; cite{rajon2013evolution}), epigenetic effects (e.g., cite{richards2012invasion}; cite{mesoudi2013non}), different forms of biased transmission citep{dalton2013biased}, and not fully appreciated effects of several kind of stochastic events (e.g.,cite{lenormand2009stochasticity}; cite{vogt2015stochastic}). Thus in the need of a more comprehensive “theory of variation,” moving forward from the limited concept of developmentally biased variation citep{fusco2015new}, this work is an exploration of the origin and evolution of a specific feature of the genotype-phenotype map, namely phenotypic robustness, or mutational robustness, (also known as genetic canalization citep{gibson2000canalization}. Phenotype mutational robustness, from now simply “phenotypic robustness”, is generally referred to as the ability of a phenotype to resist to mutational perturbations at the genetic level, stemming from the fact that multiple genotypes can encode the same phenotype. This corresponds to some extent to Kimura’ s citep{kimura1968evolutionary} claim that much of genotypic change in evolution is selectively neutral (mutations responsible for an effect on fitness are only a small minority). Empirically, the observation that RNA and protein structures are more conserved during evolution than their sequences indicates that most point mutations are neutral. In other words, only a minority of sites is conserved in sequences evolved from a single ancestor, indicating a high level of degeneracy in genotype-phenotype maps. Such mutational robustness has been observed in biological RNA structures citep{huynen1993multiple}, simulations of the evolution of RNA secondary structure citep{huynen1994pattern}, ribozymes and living organisms citep{rigato2016enhancing}.


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