Endosymbionts, eukaryotes, and evolutionary transitions

Abstract

How did biological organisms become so complex? The ‘major transitions in evolution’ offer a conceptual framework to understand the emergence of different scales of organisation in biology. In this view, the evolution of complex life has taken place via a sequence of evolutionary transitions – in each transition, cooperation between different individuals leads to the emergence of a more complex, integrated entity – a higher level of organisation. A particularly important transition is the evolution of obligate endosymbiosis. Highly evolved endosymbioses made possible the evolution of eukaryotes, as well as many other astonishing associations in, for example, the insect world. In this thesis I use evolutionary game theory, more specifically the theory of adaptive dynamics, to study the evolution of such long-term interspecific associations. I derive explicit analytical criteria, and show how the incorporation of more biological realism affects these insights and generates novel evolutionary phenomena. The main result is a robust demonstration that mutual dependence between the host and symbiont evolves faster than their reproductive cohesion i.e. their investment in synchronised reproduction. This implies that symbioses in nature are more likely to be at a higher level of mutual dependence than reproductive cohesion. These predictions have implications for our understanding of symbioses, evolutionary transitions in general, and are experimentally verifiable. In summary, I show the utility of theoretical methods in studying symbiosis, and suggest ways forward to fill gaps that this work uncovers.