On the stochastic evolution of finite populations

TL;DR

This paper systematically studies Markov chain models for two-type population evolution, extending known results for Moran and Wright-Fisher processes within a broader Kimura class, and explores fixation probabilities and their properties.

Contribution

It introduces the Kimura class of Markov chains, generalizes fixation probability results, and characterizes conditions for increasing fixation probabilities in finite populations.

Findings

Many results for Moran and Wright-Fisher processes extend to Kimura class.

A necessary and sufficient condition for increasing fixation probability is identified.

Any fixation probability without trivial fixation can be realized by some Wright-Fisher process.

Abstract

This work is a systematic study of discrete Markov chains that are used to describe the evolution of a two-types population. Motivated by results valid for the well-known Moran (M) and Wright-Fisher (WF) processes, we define a general class of Markov chains models which we term the Kimura class. It comprises the majority of the models used in population genetics, and we show that many well-known results valid for M and WF processes are still valid in this class. In all Kimura processes, a mutant gene will either fixate or become extinct, and we present a necessary and sufficient condition for such processes to have the probability of fixation strictly increasing in the initial frequency of mutants. This condition implies that there are WF processes with decreasing fixation probability --- in contradistinction to M processes which always have strictly increasing fixation probability. As a by-product, we show that an increasing fixation probability defines uniquely an M or WF process which realises it, and that any fixation probability with no state having trivial fixation can be realised by at least some WF process. These results are extended to a subclass of processes that are suitable for describing time-inhomogeneous dynamics. We also discuss the traditional identification of frequency dependent fitnesses and pay-offs, extensively used in evolutionary game theory, the role of weak selection when the population is finite, and the relations between jumps in evolutionary processes and frequency dependent fitnesses.