Evolutionary accessibility in tunably rugged fitness landscapes

TL;DR

This paper explores how the structure of fitness landscapes affects evolutionary pathways, using the Kauffman LK-model to analyze accessibility and basin of attraction, revealing complex behaviors depending on genetic interactions.

Contribution

It introduces a detailed analysis of accessibility measures in tunably rugged fitness landscapes, highlighting non-monotonic behaviors at specific interaction levels.

Findings

Accessibility generally increases with genotype size L.

Accessibility decreases as the number of interactions K increases.

Non-monotonic accessibility behavior observed at K=1 and 2.

Abstract

The adaptive evolution of a population under the influence of mutation and selection is strongly influenced by the structure of the underlying fitness landscape, which encodes the interactions between mutations at different genetic loci. Theoretical studies of such landscapes have been carried out for several decades, but only recently experimental fitness measurements encompassing all possible combinations of small sets of mutations have become available. The empirical studies have spawned new questions about the accessibility of optimal genotypes under natural selection. Depending on population dynamic parameters such as mutation rate and population size, evolutionary accessibility can be quantified through the statistics of accessible mutational pathways (along which fitness increases monotoncially), or through the study of the basin of attraction of the optimal genotype under greedy (steepest ascent) dynamics. Here we investigate these two measures of accessibility in the framework of Kauffman's $LK$-model, a paradigmatic family of random fitness landscapes with tunable ruggedness. The key parameter governing the strength of genetic interactions is the number $K$ of interaction partners of each of the $L$ sites in the genotype sequence. In general, accessibility increases with increasing genotype dimensionality $L$ and decreases with increasing number of interactions $K$. Remarkably, however, we find that some measures of accessibility behave non-monotonically as a function of $K$, indicating a special role of the most sparsely connected, non-trivial cases K=1 and 2. The relation between models for fitness landscapes and spin glasses is also addressed.