Paper detail

Two-level Fisher-Wright framework with selection and migration: An approach to studying evolution in group structured populations

A framework for the mathematical modeling of evolution in group structured populations is introduced. The population is divided into a fixed large number of groups of fixed size. From generation to generation, new groups are formed that descend from previous groups, through a two-level Fisher-Wright process, with selection between groups and within groups and with migration between groups at rate $m$. When $m=1$, the framework reduces to the often used trait-group framework, so that our setting can be seen as an extension of that approach. Our framework allows the analysis of previously introduced models in which altruists and non-altruists compete, and provides new insights into these models. We focus on the situation in which initially there is a single altruistic allele in the population, and no further mutations occur. The main questions are conditions for the viability of that altruistic allele to spread, and the fashion in which it spreads when it does. Because our results and methods are rigorous, we see them as shedding light on various controversial issues in this field, including the role of Hamilton's rule, and of the Price equation, the relevance of linearity in fitness functions and the need to only consider pairwise interactions, or weak selection. In this paper we analyze the early stages of the evolution, during which the number of altruists is small compared to the size of the population. We show that during this stage the evolution is well described by a multitype branching process. The driving matrix for this process can be obtained, reducing the problem of determining when the altruistic gene is viable to a comparison between the leading eigenvalue of that matrix, and the fitness of the non-altruists before the altruistic gene appeared. This leads to a generalization of Hamilton's condition for the viability of a mutant gene.