Séminaire Philbio - 01 - Johannes Martens

Johannes Martens, Université Catholique de Louvain (UCL) - Fraternal vs. egalitarian transitions in individuality: two processes, one concept?

In the biological literature, a “major transition in individuality” refers to the process by which a new population of biological individuals emerges from a population of entities that were previously able to undergo natural selection. During this process, the lower level entities lose their status as units of selection, and become parts of the higher level individuals (Buss 1987; Michod 1999).

Two sorts of transition are usually distinguished by evolutionary biologists, namely fraternal and egalitarian transitions (Queller 1997). In a fraternal transition, genetical relatedness plays a key role in the “transfer” of individuality from the lower level to the higher level, whereas in an egalitarian transition, the “transfer” depends on the complementarity of the partners, and on the existence of a common reproductive fate between them. The evolution of multicellular organisms is often cited as a paradigmatic instance of fraternal transition, while some symbiotic alliances – such as the eukaryote cell (which resulted from the integration of two or more prokaryotes) – are commonly regarded as bona fide outcomes of egalitarian transitions.

In the biological and philosophical literature, the term “major transition in individuality” is equally applied to describe fraternal and egalitarian events. Yet, significant differences exist between the kinds of organisation that have emerged from these evolutionary processes. In this talk, our goal will be to shed light on the nature of these differences, and to see whether the same notion of biological individuality can be used to account for the higher level outcomes of both fraternal and egalitarian transitions. We will further envisage some of the main implications/limits of a general evolutionary approach for our current understanding of endosymbioses.