The Evolutionary Ecology of Mutualism

Mutualism, cooperation between different species, is wide-spread in nature. From bees pollinating plants to bacteria aiding digestion in the human gut: mutualism is essential for life on earth. But how does mutualism evolve? And what mechanisms keep mutualisms stable over evolutionary time and prevent mutualistic partners from parasitising each other? Here, these questions are explored using both computer models that mimic evolution and a detailed ecological study of one particular kind of mutualism, farming mutualism, in which one species ‘farms’ another species for food. The underground Yellow meadow ant tends various root aphids in its nest for ‘milk’ (the sugary honeydew that aphids excrete) and ‘meat’ (by eating the aphids). In return, the ants build aphid chambers in their nest and protect the aphids actively from predators. My detailed studies of this farming mutualism on the isle of Schiermonnikoog (NL) surprisingly showed that the most common species of root aphids reproduce predominantly clonally and that they rarely disperse. The studies also show that in most aphid chambers only a single clone of aphids occurs, belonging to a single species. Interestingly, this latter example of ‘monoculture farming’ in combination with clonal reproduction of the ‘crop’ or ‘cattle’ is frequently observed in farming mutualisms. In addition, the computer simulations show that cooperative behaviour can evolve under certain conditions under which partners are able to choose with whom to cooperate. Moreover, these models show that mutualism can readily evolve when interactions are initially based on by-products, for example the honeydew of aphids.