Transitions in Levels of Organization: Lessons from Social and Colonial Spiders

In the origin of multicellular organisms or social groups, a dichotomy emerges between systems where the parts retain some autonomy, referred to as modular, and more integrated unitary systems. I explore the critical roles of ecology and geometry in determining whether groups form and the degree of integration they develop. I argue that these are essential considerations above and beyond whether groups originate from a single cell or a single inseminated female. I consider these points through the lens of social and colonial spiders, which represent early transitions to either unitary or modular systems, respectively. By allowing or constraining cooperation, I suggest that the geometry of their webs—irregular tridimensional versus orbicular—determines the degree of integration of their groups, their scaling properties, their population structure, and their long-term evolutionary fate. Ecology, on the other hand, determines the need or opportunity for groups to form. I extend these lessons to other social systems and levels of organization.