Mutualism provides a basis for biodiversity in eco-evolutionary community assembly.

Unveiling the ecological and evolutionary mechanisms underpinning the assembly of stable and complex ecosystems is a main focus of community ecology. Ecological theory predicts the necessity of structural constraints on the network of species interactions to allow for growth and persistence of multi-species communities. However, the mechanisms behind their emergence are not well understood. An understanding of how the coexistence of diverse species interaction types could influence the development of complexity and how a persistent composition of interactions could arise in nature is needed. Using an eco-evolutionary model, we investigate the assembly of complex species interaction networks with multiple interaction types and its consequences for ecosystem stability. Our results show that highly mutualistic communities promote complex and stable network configurations, thus resulting in a positive complexity-stability relationship. We show that evolution by speciation enhances the emergence of such conditions compared to a purely ecological assembly scenario of repeated invasions by migrating species. Furthermore, communities evolved in isolation promote a disproportionately higher complexity and a larger diversity of outcomes. Our results produce valuable theoretical insight into the mechanisms behind the emergence of ecological complexity and into the roles of mutualism and speciation in community formation.