  The evolutionary dynamics of molecular populations are strongly dependent on
the structure of genotype spaces. The map between genotype and phenotype
determines how easily genotype spaces can be navigated and the accessibility of
evolutionary innovations. In particular, the size of neutral networks
corresponding to specific phenotypes and its statistical counterpart, the
distribution of phenotype abundance, have been studied through multiple
computationally tractable genotype-phenotype maps. In this work, we test a
theory that predicts the abundance of a phenotype and the corresponding
asymptotic distribution (given the compositional variability of its genotypes)
through the exact enumeration of several GP maps. Our theory predicts with high
accuracy phenotype abundance, and our results show that, in navigable genotype
spaces ---characterised by the presence of large neutral networks---, phenotype
abundance converges to a log-normal distribution.
