John Maynard Smith Prize Lecture 1997

Facilitation and constraints in the evolution of mutualism?

Marie-Charlotte Anstett

CNRS-CEFE, 1919 Route de Mende, F-34293 Montpellier Cedex 5, France. E-mail:


Parasitism is the ancestral state of most mutualisms. What kinds of traits facilitate the transition from an antagonistic to a mutually beneficial interaction? The only well formalised and tested scenario for the origin of mutualism is based on the evolution of vertical transmission of parasites (from parents to offspring Yamamura, 1996), which leads to reduced virulence and sometimes to the evolution of mutualism. However, this scenario can apply only to symbiotic mutualisms, and even these include examples in which vertical transmission does not occur. For these, formalised models are lacking. What other kinds of traits facilitate the evolution of mutualism? Can we identify traits, maintained by selection on other functions, that independently in different lineages acquire the same novel function in a particular type of mutualism? If such "pre-adaptations" exist, what factors intervene to alter the selection pressures acting on them and shape them as new adaptations? How recurrent and predictable is the evolution of mutualism?

Small differences in traits already present at the origin of the mutualism may lead to differences in how the mutualism functions and how it evolves. "Constraint" is the flip side of "pre-adaptation." While constraints are usually envisaged to limit the range of evolutionary possibilities, constraints may also open evolutionary pathways that are otherwise not possible. For some mutualisms, evolutionary stability appears to be based on a coevolutionary equilibrium between trait values for the two mutualists. In other cases, however, the interaction appears to be stabilised by constraints imposed by preexisting traits of one species that the associated species cannot evolve to overcome. These points will be developed using as examples the fig/ fig wasp pollination mutualism and protective ant/plant interactions.



Last updated April 09, 1997. For suggestions or comments please send an e-mail.