Obligate multicellularity circumvents population genetic barriers to collective-level adaptation

Complex multicellularity has evolved in just five lineages (animals, plants, brown algae, red algae, and fungi) and in each case, these organisms develop clonally and are obligately multicellular. While prior work has shown that clonal development plays a critical role in the evolution of complex multicellularity, none has disentangled this from the impact of obligate vs facultative multicellular life cycles. Here we use experimental evolution with engineered snowflake yeast (Saccharomyces cerevisiae) to directly test how life cycle structure affects multicellular adaptation. We created isogenic strains capable of switching between unicellular and clonal multicellular phases, then evolved populations for 192 days under obligately multicellular, facultatively multicellular, and obligately unicellular regimes. Obligately multicellular populations rapidly evolved larger size, primarily driven by a whole genome duplication, in all five replicates. Facultative populations showed dramatically constrained evolution, with tetraploidy evolving in only 2/10 facultative populations despite experiments demonstrating that it is strongly beneficial across the full life cycle. Mathematical modeling reveals the mechanistic basis for this constraint: facultative life cycles create establishment barriers through two population genetic effects. Group formation dramatically reduces the number of units of selection, making beneficial multicellular mutations vulnerable to drift. This asymmetry in population size between life cycle phases also allows cell-level selection to overpower group-level selection, eliminating mutations that provide group-level benefits but carry cell-level costs. These findings demonstrate that obligate multicellularity circumvents fundamental population genetic barriers to collective-level adaptation, helping explain why complex multicellularity has evolved exclusively in obligately multicellular lineages, and suggesting similar constraints may operate in other evolutionary transitions in individuality.