Araujo, G.; Lurgi, M.

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 to allow the growth of complexity in assembling communities. There is still little understanding of how the coexistence of diverse interaction types could drive complexity and how an ideal composition could arise in nature. To address these questions, we propose an ecological model with mixed interaction types that implements evolutionary assembly by speciation. Our results show that highly mutualistic communities are drivers of complexity and promoters of consumer-resource interactions, and that an evolutionary process is required to produce such a condition. Moreover, an evolutionary assembly generates a diversity of outcomes and promotes two distinct types of complexity depending on speciation constraints. These two types exhibit efforts towards either more richness or connectivity, in agreement with patterns previously observed in microbial communities. Our results produce invaluable theoretical insight into the mechanisms behind the emergence of complexity and into the roles of mutualism and speciation on community formation.