The pea aphid complex as a model of ecological speciation

1. Host‐specialised races of plant‐feeding insects are particularly informative models in the study of ecological speciation, that is, the evolution of reproductive isolation through divergent natural selection. However, within the enormous diversity of phytophagous insects, the mechanisms of ecological divergence have been elucidated in few host race systems.2. Here we review the literature covering speciation through host‐plant specialisation in a well‐studied model, the pea aphid complex,Acyrthosiphon pisum, which encompasses numerous biotypes that parasitise different legume host species worldwide.3. Published results are consistent with ecologically promoted reproductive isolation. Divergent host‐induced selection is pronounced across biotypes, and reflects genetic trade‐offs preventing the optimal use of multiple host plants. While these genetic trade‐offs may partly explain the unfitness of hybrids between biotypes, hybridisation occurring on plants is also limited by genetically‐based host preference, and by selection against migrants that chose unfavourable hosts. The continuum of genetic divergence displayed by 11 races and species of the pea aphid complex suggests that host races constitute an intermediate step in the speciation process, and that host specialisation may indeed lead to complete speciation. Uncertainties remain on the contribution of non‐ecological reproductive barriers to biotype divergence and on the physiological and molecular bases of host specialisation.