| Summary: | The history of life is punctuated by major transitions in individuality, when previously-independent biological units assemble into new agents. The stability of these evolutionary alliances is governed by the extent of cooperation and conflict among their constituents. In this thesis, I explore how ecology influences these social interactions. Part I investigates cooperation in arthropod societies. W. D. Hamilton's haplodiploidy hypothesis, which holds that asymmetries in sibling relatedness explain the exclusively-female worker caste of the social Hymenoptera, has fallen out of favour; instead, the preadaptation of females for rearing young is thought to explain who helps in these taxa. Analysing the evolution of paternal care, I show that this pattern of preadaptation is not itself a consequence of haplodiploidy inhibiting brood care by males. I then show how five key factors of sexual ecology - sex-specific preadaptation, labile sex allocation, sib-mating, promiscuity, and reproductive autonomy - have a major impact upon the evolution of sex-biased helping in arthropod societies. Finally, inclusive-fitness theory predicts that monogamy should promote worker sterility in insect societies, but a recent population-genetics model has challenged this prediction. I show that relaxing this model's genetic, evolutionary, and ecological assumptions supports inclusive-fitness theory. Part II investigates conflict within genomes. I show that transposons are under selection to drive their host populations extinct, and explore the transposon-host and transposon-transposon interactions that might prevent this outcome. While neither selection against transposons' deleterious effects nor exploitation by parasites can plausibly prevent extinction, I show that host suppression of transposons can; however, this is only stable in the long term if suppression makes transposition costlier. I conclude that complex life could not exist without the active suppression of genetic conflict. Overall, I argue that ecology explains patterns of cooperation and conflict across the major transitions.
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