Life is hierarchically structured, with replicating entities nested within higher order self-replicating structures. Such hierarchical structure reflects a series of major evolutionary transitions in which lower order self-replicating entities have been subsumed within higher order structures. Typically this involves the lower level entity "giving up its right to autonomous replication" and with this "sacrifice" comes enslavement to the "Needs" of the higher order "corporate Body". Posed in these terms it is difficult to see how evolutionary transitions unfold; how selection might shift levels and why life is hierarchically structured. Pressing to the heart of the problem, the challenge is to explain how Darwinian properties emerge from non-Darwinian entities by non-Darwinian means.

Paul B. Rainey is Director of the Department of Microbial Population Biology at the MPI for Evolution Biology as well as Group Leader of the Laboratoire de Génétique de l’Evolution at The ESPCI Paris and Visiting Fellow at CAS.