Bacterial collectives : from group formation to units of selection

The natural world is full of bacterial collectives, ranging from transient aggregations to highly integrated groups. These structures can display multicellular‑like features but typically do not reproduce as groups, and thus are not units of selection in a paradigmatic Lewontin-sense. In the framework of evolutionary transitions in individuality, such collectives must first emerge and then acquire Darwinian individuality, gaining variation, reproduction, and heredity at the collective level. Populations of Pseudomonas fluorescens SBW25 provide a useful model: they readily evolve mat‑forming lineages at the air‑liquid interface in static microcosms. Although beneficial in the short term, these mats lack intrinsic group‑level reproduction. Their incomplete multicellularity makes them ideal for studying early steps toward multicellularity. In this thesis, I follow the evolutionary trajectory of SBW25 multicellular mats, from their emergence as multicellular groups and their underlying genetic constraints (Part I) to their Darwinization via ecological scaffolding with the imposition of a life cycle regime with a lineage-level death-birth process (Part II).

Together, these results show that group emergence and Darwinization are tightly linked through genetic architecture and ecological context. During group emergence, genetics shape whether phenotypes can converge despite molecular divergence and limit the pathways to alternative multicellular forms. During group Darwinization, genetics continue to influence the establishment of reliable life cycles, while ecological scaffolding defines the demographic conditions and the strength of selection at the collective level. Overall, this study shows how variation, bias, and ecological context persist across individuality stages, connecting early phenotypic diversity with the routes by which groups evolve toward Darwinian individuality.