Epistasis can lead to fragmented neutral spaces and contingency in evolution

TL;DR

This paper investigates how epistatic interactions create fragmented neutral spaces in RNA folding, revealing that these structures influence evolvability and contingency in evolution.

Contribution

It demonstrates that epistasis causes neutral networks to fragment into multiple components, affecting evolutionary pathways and robustness.

Findings

Neutral networks fragment into multiple components due to epistasis

Components vary in evolvability and robustness

Fragmentation increases contingency in evolution

Abstract

In evolution, the effects of a single deleterious mutation can sometimes be compensated for by a second mutation which recovers the original phenotype. Such epistatic interactions have implications for the structure of genome space - namely, that networks of genomes encoding the same phenotype may not be connected by single mutational moves. We use the folding of RNA sequences into secondary structures as a model genotype-phenotype map and explore the neutral spaces corresponding to networks of genotypes with the same phenotype. In most of these networks, we find that it is not possible to connect all genotypes to one another by single point mutations. Instead, a network for a phenotypic structure with $n$ bonds typically fragments into at least $2^n$ neutral components, often of similar size. While components of the same network generate the same phenotype, they show important variations in their properties, most strikingly in their evolvability and mutational robustness. This heterogeneity implies contingency in the evolutionary process.