This thesis is concerned with the major transitions view of evolution; the idea that general principles operate in the evolution of each new level of the biological hierarchy (Bourke, 2011). We discuss the theoretical background of this field, focussing on inclusive fitness theory and multi-level selection theory, different approaches to analysing the selection of traits. Many of the commonalities between different transitions are dependent on whether they occur within or between species, and whether relatedness is absent (`egalitarian') or present (`fraternal') (Queller, 1997). Altruism underpins fraternal transitions, and mutually beneficial behaviour underpins egalitarian transitions (Bourke, 2011). We focus on several different models relating to this four-way decomposition.
Firstly, we focus on arguments that between-species donation may amount to between-species altruism; this has been a point of contention within the literature (Fletcher and Doebeli, 2009; Gardner et al., 2011; Wyatt et al., 2013). We discuss both deterministic (resting on an assumption of quasi-linkage equilibrium) and stochastic approaches to a simple model of between-species donation, finding that stable donation behaviour can evolve in the presence of assortment across all loci, but is vulnerable to unassorted modifiers. We argue that this behaviour can be interpreted as within-species altruism, using the other species as a vector for altruism, and, further, consider our models in relation to the current literature on greenbeards.
Our second model concerns maternally-transmitted sex-distorting endosymbionts. Many species, particularly insect populations, are infected by sex-distorting parasites such as the bacteria Wolbachia, which are maternally-transmitted; thus, distortion of sex ratios towards the production of females may be beneficial to the symbiont. We investigate the potential for a reproductive parasite to transition towards mutualism, laying the foundation for an egalitarian transition between species; in particular, we find that population structure is key to this transition.
Finally, we discuss several potential avenues for future research; in particular, we note that the social group transformation phase of a major transition involves a number of open questions, or ideas open to further investigation.
