Muller's ratchet with overlapping generations

TL;DR

This paper develops an analytical framework for Muller's ratchet with overlapping generations using a Moran model, providing explicit expressions for the ratchet rate and quasi-stationary distribution, and confirming results numerically.

Contribution

It introduces a novel approximation scheme for the Moran model of Muller's ratchet, enabling analytical expressions for key quantities in the rare clicking regime.

Findings

Derived closed-form expressions for the ratchet rate.

Confirmed the equivalence of overlapping and non-overlapping generation models.

Provided analytical insights into the quasi-stationary distribution.

Abstract

Muller's ratchet is a paradigmatic model for the accumulation of deleterious mutations in a population of finite size. A click of the ratchet occurs when all individuals with the least number of deleterious mutations are lost irreversibly due to a stochastic fluctuation. In spite of the simplicity of the model, a quantitative understanding of the process remains an open challenge. In contrast to previous works, we here study a Moran model of the ratchet with overlapping generations. Employing an approximation which describes the fittest individuals as one class and the rest as a second class, we obtain closed analytical expressions of the ratchet rate in the rare clicking regime. As a click in this regime is caused by a rare large fluctuation from a metastable state, we do not resort to a diffusion approximation but apply an approximation scheme which is especially well suited to describe extinction events from metastable states. This method also allows for a derivation of expressions for the quasi-stationary distribution of the fittest class. Additionally, we confirm numerically that the formulation with overlapping generations leads to the same results as the diffusion approximation and the corresponding Wright-Fisher model with non-overlapping generations.