Despite the fact that ecological interactions are often described exclusively as either competitive, antagonistic or mutualistic, any interaction involves, in fact, a balance of private and overlapping interests among interacting individuals (partners). A critical question is understanding how the components of this balance can arise and change in the course of evolution. In this paper, we will present an adaptive-dynamics analysis of this problem. This is a ‘two-tier’, ecological/evolutionary approach which begins with a general model of an ecological interaction between two species. In the ecological model, any individual may, at any time, be ‘free’ or associated with a ‘partner’ to form a ‘complex’. Individuals of each species are characterized by three adaptive traits which are expressed in the ‘complex’ stage: exploitation of partner, response to exploitation by partner, and investment into common good. The model’s construction allows for a rigorous definition of an individual’s ‘private interest’ and of the ‘common good’ of the association. The association may be ‘profitable’ for either species or both. Evolution results from heritable variation in the adaptive traits and selection due to ecological interactions.

Results/Conclusions

Evolution can drive changes in profitabilities and favor the emergence of a common good. ‘Alignment of interest’ occurs when profitabilities increase simultaneously; decrease in one profitability indicates a ‘conflict of interest’. ‘Cooperation’ can evolve in two ways: increasing common good, or alignment of interest. Six evolutionary transitions can take place. (i) Evolution toward ‘mutualistic symbiosis’ occurs when cooperation evolves through renounced private interests, bolstered common good and interests alignment. (ii) ‘Exploiter-victim mutualism’ evolves when common good is bolstered and interests are aligned although one partner increases its private interest. (iii) ‘Mutualism through reciprocal exploitation’ is similar except that the private interests of both partners increase. (iv) ‘Conflictual cooperation’ evolves with bolstering common good; at least one partner renounces private interest, yet the partners’ interests do not align. (v) ‘Cooperation by interest alignment’ evolves when interests become aligned whereas private interests are not renounced and common good is not bolstered. (vi) Competition is the evolutionary outcome when there is neither common good bolstering nor interests alignment. The same system may experience several transitions, forward or backward, during its evolutionary history. Thus, coevolution in the ecological landscape of competition, antagonism and mutualism is a much richer and more dynamical process than suggested by the traditional view of directional and irreversible selection toward or against cooperation.