Quasispecies theory for finite populations

TL;DR

This paper develops stochastic models for finite populations in quasispecies theory, revealing large fluctuations and the mitigating effects of horizontal gene transfer on mutation accumulation, with analytical and numerical insights.

Contribution

It introduces finite-population stochastic formulations of the Crow-Kimura and Eigen models, including back mutations and horizontal gene transfer effects.

Findings

Large fluctuations in population numbers in finite models.

Horizontal gene transfer significantly reduces mutation accumulation.

Population sizes needed for convergence are often larger than natural populations.

Abstract

We present stochastic, finite-population formulations of the Crow-Kimura and Eigen models of quasispecies theory, for fitness functions that depend in an arbitrary way on the number of mutations from the wild type. We include back mutations in our description. We show that the fluctuation of the population numbers about the average values are exceedingly large in these physical models of evolution. We further show that horizontal gene transfer reduces by orders of magnitude the fluctuations in the population numbers and reduces the accumulation of deleterious mutations in the finite population due to Muller's ratchet. Indeed the population sizes needed to converge to the infinite population limit are often larger than those found in nature for smooth fitness functions in the absence of horizontal gene transfer. These analytical results are derived for the steady-state by means of a field-theoretic representation. Numerical results are presented that indicate horizontal gene transfer speeds up the dynamics of evolution as well.