Cooperation

Cooperation is integral to the origin, persistence, and expansion of biological organization. All macroscopic organisms require cooperation among the cells in their bodies, and many (including bees, ants, and ourselves) work together to construct (mostly) cooperative societies. However, for all its importance, cooperation is susceptible to exploitation by selfish individuals. Left unchecked, these ‘cheaters’ can impair or even destroy cooperative groups, including organisms (cancer) and societies.

How can cooperation survive (let alone thrive) under the rules of Darwinian competition? This and related questions have challenged evolutionary biologists to look beyond highly simplified, individualistic conceptions of natural selection (“nature red in tooth and claw”, “survival of the fittest”, etc.), beginning with Darwin himself.

Multi-level cooperation and competition can generate complex, self-organized social environments that maintain diverse social 'strategies'. Selection to avoid or suppress selfishness may impose strict limits on cooperation among distinct lineages. However, avoiding non-kin also imposes opportunity costs on discriminating genotypes, which may outweigh the negative effects of cooperating with free-riders.

Experimental evolution allows us to observe the origin and fate of cooperation in diverse populations. Multicellular clusters of the yeast Kluyveromyces lactis evolved under intense selection to rapidly settle to the bottom of cultures, a simple way to simulate selection for multicellularity in the laboratory. Interestingly, evolved multicellular clusters cooperate with unrelated neighbors to form massive social groups ('flocs') that dramatically increase settling speeds, even though this indiscriminate cooperation also benefits free-riding (unicellular) competitors. Each cluster is made of clonemates, so why would clusters continue to cooperate with non-kin, including free-riders? The videos below suggest an answer: individual cluster speeds (indicated by the colors of the 'tracks') are higher within flocs (left), whereas cluster speeds dramatically decrease when inter-cluster aggregation is chemically disrupted (right). In this system, it is likely that missing out on positive interactions with unrelated cooperators represents a greater Darwinian 'threat' than that posed by the occasional free-rider.