As natural selection acts on individual organisms the evolution of costly cooperation between microorganisms is an intriguing phenomenon. Introduction of spatial structure to privatize exchanged molecules can explain the evolution of cooperation. However, in many natural systems cells can also grow to low cell concentrations in the absence of these exchanged molecules, thus showing “cooperation-independent background growth”. We here serially propagated a synthetic cross-feeding consortium of lactococci in the droplets of a water-in-oil emulsion, essentially mimicking group selection with varying founder population sizes. The results show that when the growth of cheaters completely depends on cooperators, cooperators outcompete cheaters. However, cheaters outcompete cooperators when they can independently grow to only ten percent of the consortium carrying capacity. This result is the consequence of a probabilistic effect, as low founder population sizes in droplets decrease the frequency of cooperator co-localization. Cooperator-enrichment can be recovered by increasing the founder population size in droplets to intermediate values. Together with mathematical modelling our results suggest that co-localization probabilities in a spatially structured environment leave a small window of opportunity for the evolution of cooperation between organisms that do not benefit from their cooperative trait when in isolation or form multispecies aggregates.

由于自然选择作用于个体生物体，因此微生物之间代价高昂的合作进化是一个有趣的现象。引入空间结构使交换的分子私有化可以解释合作的演变。然而，在许多自然系统中，在没有这些交换分子的情况下，细胞也可以生长到低细胞浓度，从而显示出“不依赖合作的背景生长”。我们在这里在油包水乳液的液滴中连续繁殖了一个合成的交叉喂养乳球菌群落，基本上模仿了具有不同创始人种群大小的群体选择。结果表明，当作弊者的成长完全依赖于合作者时，合作者胜过作弊者。然而，当作弊者可以独立增长到仅占联盟承载能力的 10% 时，他们就会在竞争中胜过合作者。这个结果是概率效应的结果，因为液滴中较低的创始人种群大小降低了合作者共定位的频率。合作者富集可以通过将液滴中的创始人种群大小增加到中间值来恢复。连同数学模型，我们的结果表明，空间结构化环境中的共定位概率为生物体之间的合作进化留下了一个小机会窗口，这些生物体在孤立或形成多物种聚集体时无法从其合作特性中获益。由于液滴中较低的创始人人口规模降低了合作者共同定位的频率。合作者富集可以通过将液滴中的创始人种群大小增加到中间值来恢复。连同数学模型，我们的结果表明，空间结构化环境中的共定位概率为生物体之间的合作进化留下了一个小机会窗口，这些生物体在孤立或形成多物种聚集体时无法从其合作特性中获益。由于液滴中较低的创始人人口规模降低了合作者共同定位的频率。合作者富集可以通过将液滴中的创始人种群大小增加到中间值来恢复。连同数学模型，我们的结果表明，空间结构化环境中的共定位概率为生物体之间的合作进化留下了一个小机会窗口，这些生物体在孤立或形成多物种聚集体时无法从其合作特性中获益。