 The study evolved two replicate populations of clonally reproducing snowflake yeast and aggregative flock yeast, with daily selection for rapid growth and sedimentation, finding that aggregative flock yeast obtained nearly all their increased fitness from faster growth while clonal snowflake yeast mainly benefited from higher group-dependent fitness. Genome sequencing and mathematical modelling showed that genetic bottlenecks in a clonal life cycle drive much higher rates of genetic drift, highlighting the central role that early multicellular life cycles play in the process of multicellular adaptation. This article was authored by Jennifer T. Pence, Catherine MacGillivray, James G. Dubose, and others.