Endosymbiotic mutualism can constrain host diversity and evolved complexity

Coevolutionary arms races between hosts and parasites are known to promote the evolution of complex traits in hosts. However, the evolutionary effects of mutualistic endosymbionts (symbionts that live inside a host) are less well understood. Here, we use populations of self-replicating computer programs (digital organisms) to investigate the effects of trait matching mutualisms on evolution. We replicate previous findings showing that parasitic endosymbionts increase host diversity and promote the evolution of complex traits. Contrastingly, we find that coevolution with mutualistic symbionts decreases host diversity and can constrain the evolution of the host species by imposing stabilizing selection to maintain symbiosis. Under strict trait matching conditions, mutualistic symbionts in our experiments often undergo a form of egalitarian transition in individuality with their hosts, losing the ability to reproduce independent of their host. The constraining effects of mutualism depend on several factors: environmental benefits available to hosts that forgo mutualism, strength of mutualistic interactions, and restrictions on host-mutualist compatibility. Strikingly, when we experimentally remove mutualists from an evolutionarily constrained host population, hosts become unconstrained, which rapidly changes the trajectory of host evolution.