Speciation by symbiosis.

In the Origin of Species, Darwin struggled with how continuous changes within a species lead to the emergence of discrete species. Molecular analyses have since identified nuclear genes and organelles that underpin speciation. In this review, we explore the microbiota as a third genetic component that spurs species formation. We first recall Ivan Wallin's original conception from the early 20th century on the role that bacteria play in speciation. We then describe three fundamental observations that justify a prominent role for microbes in eukaryotic speciation, consolidate exemplar studies of microbe-assisted speciation and incorporate the microbiota into classic models of speciation.