Theoretical Biology Group, Biology Department, Utrecht University, Utrecht, The Netherlands.
Centre for Complex Systems Studies, Utrecht University, Utrecht, The Netherlands.
PLoS Comput Biol. 2022 Oct 25;18(10):e1010612. doi: 10.1371/journal.pcbi.1010612. eCollection 2022 Oct.
Theories on the evolutionary origins of altruistic behavior have a long history and have become a canonical part of the theory of evolution. Nevertheless, the mechanisms that allow altruism to appear and persist are still incompletely understood. It is well known, however, that the spatial structure of populations is an important determinant. In both theoretical and experimental studies, much attention has been devoted to populations that are subdivided into discrete groups. Such studies typically imposed the structure and dynamics of the groups by hand. Here, we instead present a simple individual-based model in which altruistic organisms spontaneously self-organize into spatially separated colonies that themselves reproduce by binary fission and hence behave as Darwinian entities in their own right. Using software to automatically track the rise and fall of colonies, we are able to apply formal theory on multilevel selection and thus quantify the within- and among-group dynamics. This reveals that individual colonies inevitably succumb to defectors in a within-colony "tragedy of the commons". Even so, altruism persists in the population because more altruistic colonies reproduce more frequently and drive less altruistic ones to extinction. Evidently, the colonies promote the selection of altruism but in turn depend on altruism for their existence; the selection of altruism hence involves a kind of evolutionary bootstrapping. The emergence of the colonies also depends crucially on the length scales of motility, altruism, and competition. This reconfirms the general relevance of these scales for social evolution, but also stresses that their impact can only be understood fully in the light of the emergent eco-evolutionary spatial patterns. The results also suggest that emergent spatial population patterns can function as a starting point for transitions of individuality.
关于利他行为进化起源的理论由来已久，已成为进化理论的一个规范部分。然而，允许利他行为出现和持续的机制仍不完全清楚。然而，众所周知，种群的空间结构是一个重要的决定因素。在理论和实验研究中，人们非常关注划分为离散群体的种群。这些研究通常通过手工来施加群体的结构和动态。在这里，我们提出了一个简单的基于个体的模型，其中利他生物会自发地自我组织成空间上分离的殖民地，这些殖民地本身通过二分法繁殖，因此本身就表现为达尔文实体。我们使用软件自动跟踪殖民地的兴衰，从而能够应用关于多层次选择的正式理论，并因此量化群体内和群体间的动态。这表明，个体殖民地不可避免地会在群体内的“公共地悲剧”中屈服于叛徒。即便如此，利他主义在种群中仍然存在，因为更利他主义的殖民地繁殖更频繁，并导致不那么利他主义的殖民地灭绝。显然，殖民地促进了利他主义的选择，但反过来又依赖于利他主义的存在；利他主义的选择因此涉及一种进化的自举。殖民地的出现也严重依赖于运动、利他主义和竞争的长度尺度。这再次证实了这些尺度对社会进化的普遍相关性，但也强调了只有在新兴的生态进化空间模式的背景下，才能充分理解它们的影响。研究结果还表明，新兴的空间种群模式可以作为个体性转变的起点。
