Clonal interference and Muller's ratchet in spatial habitats

TL;DR

This paper investigates how spatial structure influences evolutionary dynamics like clonal interference and Muller's ratchet, revealing universal fitness distribution shapes and phase transitions akin to surface growth and percolation models.

Contribution

It provides precise predictions for fitness distributions in spatial habitats and links evolutionary processes to surface growth and percolation theories, highlighting new universal behaviors.

Findings

Fitness distribution in 1D habitats is non-Gaussian and universal.

Rate of fitness decline remains finite in large habitats under certain conditions.

Transition in Muller's ratchet is governed by directed percolation.

Abstract

Competition between independently arising beneficial mutations is enhanced in spatial populations due to the linear rather than exponential growth of clones. Recent theoretical studies have pointed out that the resulting fitness dynamics is analogous to a surface growth process, where new layers nucleate and spread stochastically, leading to the build up of scale-invariant roughness. This scenario differs qualitatively from the standard view of adaptation in that the speed of adaptation becomes independent of population size while the fitness variance does not. Here we exploit recent progress in the understanding of surface growth processes to obtain precise predictions for the universal, non-Gaussian shape of the fitness distribution for one-dimensional habitats, which are verified by simulations. When the mutations are deleterious rather than beneficial the problem becomes a spatial version of Muller's ratchet. In contrast to the case of well-mixed populations, the rate of fitness decline remains finite even in the limit of an infinite habitat, provided the ratio $U_d/s^2$ between the deleterious mutation rate and the square of the (negative) selection coefficient is sufficiently large. Using again an analogy to surface growth models we show that the transition between the stationary and the moving state of the ratchet is governed by directed percolation.