Abstracts (first author)
Major transitions, the evolution of multicellularity and the size-complexity hypothesis
The evolution of multicellularity is a prime example of a major transition leading to the evolution of individuality at a new hierarchical level [1-3]. As such, it exhibits many parallels with other major transitions, including the origin of eusociality in insects [4, 5]. Inclusive fitness theory represents a powerful tool for analysing the major transitions [4-6]. I discuss the evolution of multicellularity, including the evolution of a germline, in light of inclusive fitness theory. For example, the fact that most origins of multicellularity occurred via daughter cells remaining stuck to parent cells (subsocial route), and the likelihood that the first multicellular organisms had low cell numbers, suggest that cells within early multicellular organisms were clonal and exhibited few somatic mutations. This in turn suggests a near-identity of inclusive-fitness interests, implying that a germline would have evolved in such organisms not to prevent disruption from selfish cell lineages but to increase efficiency through a reproductive division of labour [4, 5]. The size-complexity hypothesis proposes that, as multicellular organisms grew larger, the increasing incidence of somatic mutations promoted the evolution of a segregated germline [5, 7]. The hypothesis predicts an association across taxa between a segregated germline and high cell number. I discuss evidence for this association, consequences of a segregated germline, and parallels with the evolution of eusocial insects.
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