On the evolutionary origins of host–microbe associations

Significance Animals can provide benefits to their associated microbes—and these can, in turn, positively affect their hosts. But how do such mutually beneficial associations arise in the first place? In particular, when animal and microbe initially have independent lifestyles, this is not clear. By developing a model of animal and microbial life cycles on patchy habitats, we show how their overlapping ecologies of development and dispersal can lead to the enrichment of certain microbes in the dispersing animals, even in the absence of specific mutualistic benefits. This enrichment can then set the stage for the evolution of more specific host–microbe associations, which also implies that host enrichment per se is not an indicator of a beneficial host–microbe symbiosis.