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Dispersal, fitness decoupling, germ line, group selection, soma
Abstract:
The evolutionary transition to multicellularity has occurred on numerous occasions, but transi-
tions to complex life forms are rare. Here, using experimental bacterial populations as proxies for
nascent multicellular organisms, we manipulate ecological factors shaping the evolution of groups.
Groups were propagated under regimes requiring reproduction via a life cycle replete with devel-
opmental and dispersal (propagule) phases, but in one treatment lineages never mixed, whereas in
a second treatment, cells from different lineages experienced intense competition during the disper-
sal phase. The latter treatment favoured traits promoting cell growth at the expense of traits
underlying group fitness – a finding that is supported by results from a mathematical model. Our
results show that the transition to multicellularity benefits from ecological conditions that main-
tain discreteness not just of the group (soma) phase, but also of the dispersal (germline) phase.
