The history of life on earth can be seen as a sequence of major transitions in which lower-level entities evolve to cooperate with each other and form ever higher levels of biological organisation. Genes cooperate to form genomes and cells, cells cooperate to form multicellular individuals, individual organisms cooperate to form social communities and species cooperate in mutualistic interactions. Yet the stability of higher-level cooperative units is constantly threatened by potential conflict between lower-level entities over their share in the genetic contribution to future generations. We study how this balance between conflict and cooperation shapes individual development, the social structure of populations and the dynamics of mutualistic interactions.

A fundamental developmental process is sex determination. Will an individual develop into a female, a male or both and become a hermaphrodite? What cues are used to trigger sexual development? We build models to investigate how the evolution of sex determination is affected by parent-offspring conflict, sexual conflict and environmental variability. A great advantage of studying sex determination is that model predictions are relatively easy to test by observing sex ratios in the lab or in the field. We collaborate with various national and international partners to test our models in a wide range of taxa, from tiny short-lived hermaphroditic worms to long-lived reptiles. More generally, we develop models to study the role of maternal effects and epigenetics on the evolution of individual developmental plasticity. To what extent should an individual ‘listen’ to cues transmitted by its mother or by the environment? What are the evolutionary implications of epigenetically mediated transgenerational inheritance?

Highly social species, such as humans and eusocial insects, are characterized by strong division of labour. We study under what conditions division of labour can 'self-organize' in groups of phenotypically plastic individuals. How can development of discrete casts evolve from an initially homogeneous workforce? How important is cultural evolution as opposed to genetic evolution? Mutualistic interactions between species are common and ecologically important. Yet it is still far from clear how mutualisms remain stable in view of the temptation to exploit partners. What mechanisms are expected to evolve to ensure stable cooperation? How important are partner choice and sanctions against non-cooperative partners? What is the role of horizontal versus vertical transmission of partnership? By applying evolutionary principles to these lower-level processes, those patterns and processes will be singled out that are evolutionarily stable. The integration of an evolutionary and a self-organization approach is a major challenge and one of the central goals of our research programme.