Evolutionary principles promoting cooperation

Life shows a stunning level of complexity. Understanding the emergence of this complexity and the functioning of intricate biological processes lies at the heart of modern biology. One major hallmark of biological complexity, including many forms of interacting organisms, is cooperation. Cooperating individuals are, by providing a benefit, capable to facilitate a better depletion of resources, a more efficient protection against threats, or the formation of social entities, to name but a few advantages. However, explaining cooperation is a major challenge of evolutionary theory: Why do cooperators persist if non-cooperative individuals can get away from paying the costs and benefit from faithful cooperators? This dilemma of cooperation emerges for a vast variety of life forms and on different levels of biological complexity. This thesis focuses on the ecological factors promoting the evolution of cooperation. In particular, the role of demographic fluctuations, growth-dynamics and population structure is considered. Correspondingly, this thesis is divided into three main parts, briefly introduced in the following. In the first part, the role of demographic fluctuations is studied. By the stochastic nature of the underlying birth and death events, the evolutionary dynamics of a population is always subject to demographic fluctuations. We here analyze their impact on the evolutionary outcome and compare it with the selection for fitter individuals. We focus on ‘evolutionary games’ and show that there are broad regimes of neutral-evolution where fluctuations dominate the dynamics. Furthermore, we quantify the edge of neutral evolution where fitness-differences become important and demographic fluctuations are only of minor relevance. The second part considers the coupling of evolutionary and population dynamics. Growth and decline of a population as well as its internal evolution result from the same birth and death events and thus are coupled. We introduce a stochastic model to study this coupling and discuss the relation to common evolutionary models which assume a constant population size and hence do not take this coupling into account. As a particular example, we analyze the dilemma of cooperation in a growing population. We show that the interplay of demographic fluctuations and population dynamics can drastically influence the evolutionary outcome and lead to a transient increase of cooperation. We study this increase for different typical types of microbial growth dynamics. In the third part, the evolutionary dynamics in structured populations is investigated. In particular, we consider group-structured populations where individuals are regularly assorted into new groups. The model serves as a null model to study cooperation in microbes under the permanent influence of population-bottlenecks. These can arise in nature due to life-cycles or migration events. We analyze the interplay of growth, internal evolution, and assortment dynamics and show that there are two mechanisms promoting cooperation: the group-growth and the group-fixation mechanism. The group-growth mechanism is based on the faster growth of more cooperative groups and facilitates the evolution of cooperation from one single cooperating mutant on. The group-fixation mechanism rest upon the advantage of purely cooperative groups and can lead to entirely cooperative populations in the long run. Experiments to test our predictions and their dependence on the key parameters are currently performed by Prof. Kirsten Jung and Prof. Heinrich Jung at her chair of microbiology at the Ludwig-Maximilians University Munich. The outline of this thesis is as follows. Chapter 1 gives a short introduction to evolutionary theory in general and states the broader context of this thesis. The following Chapters 2 and 3 provide an introduction to the mathematical formulations of evolutionary dynamics and the issue of cooperation, respectively. Chapters 4, 5, and 6 then consider the three main issues introduced before. Each of these chapters ends with a short discussion and a reprint of the corresponding papers and manuscripts. In Chapter 4, demographic fluctuations and the edge of neutral evolution are analyzed. Subsequently, the coupling between population dynamics and evolutionary dynamics is discussed in Chapter 5. Finally, the role of population structure is discussed in Chapter 6. It also considers microbial organisms and their life-cycles in detail.