The evolution of multicellularity set the stage for sustained increases in organismal complexity^1,2,3,4,5. However, a fundamental aspect of this transition remains largely unknown: how do simple clusters of cells evolve increased size when confronted by forces capable of breaking intracellular bonds? Here we show that multicellular snowflake yeast clusters^6,7,8 fracture due to crowding-induced mechanical stress. Over seven weeks (~291 generations) of daily selection for large size, snowflake clusters evolve to increase their radius 1.7-fold by reducing the accumulation of internal stress. During this period, cells within the clusters evolve to be more elongated, concomitant with a decrease in the cellular volume fraction of the clusters. The associated increase in free space reduces the internal stress caused by cellular growth, thus delaying fracture and increasing cluster size. This work demonstrates how readily natural selection finds simple, physical solutions to spatial constraints that limit the evolution of group size—a fundamental step in the evolution of multicellularity.

多细胞的进化为生物体复杂性的持续增加奠定了基础^1,2,3,4,5 。然而，这种转变的一个基本方面仍然很大程度上未知：当面对能够破坏细胞内键的力时，简单的细胞簇如何进化出更大的尺寸？在这里，我们展示了多细胞雪花酵母簇^6、7、8由于拥挤引起的机械应力而断裂。经过七周（约 291 代）的日常选择，雪花簇通过减少内应力的积累而进化，使其半径增加了 1.7 倍。在此期间，簇内的细胞进化得更加伸长，同时簇的细胞体积分数减少。自由空间的相关增加减少了细胞生长引起的内应力，从而延迟了断裂并增加了簇尺寸。这项工作展示了自然选择如何轻松地找到简单的物理解决方案来解决限制群体规模进化的空间限制——这是多细胞进化的基本步骤。