Our understanding of life is being transformed by the realization that evolution occurs not only through the standard processes of genetic change operating within populations, but also during evolutionary transitions in individuality (ETIs), when groups of individuals become so integrated that they evolve into new higher-level individuals. Indeed, the hierarchical organization of the living world is a consequence of a series of evolutionary transitions: from genes to gene networks to the first cell, from prokaryotic to eukaryotic cells, from cells to multicellular organisms, from asexual to sexual populations, and from solitary to social organisms. It is a major challenge to understand why (environmental selective pressures) and how (underlying genetics, population structure, physiology, and development) the basic features of an evolutionary individual, such as fitness heritability, indivisibility, and evolvability, shift their reference from the old level to the new level. In the talk, I consider the transition from unicellular to multicellular organisms, using the volvocine green algae as a model experimental system. These algae comprise both uni- and multicellular forms and various levels of individuality and complexity. How and why do groups become individuals? These are the central questions motivating our work. I consider the hypothesis that germ-soma specialization is fundamental to the conversion of cell groups into true multicellular individuals. I discuss work addressing this hypothesis using both ultimate (evolutionary) and proximate (mechanistic) approaches, employing theoretical, experimental and comparative methods.

Results/Conclusions

Our theory predicts that the trade-off between fitness components (viability and reproduction) is a major factor driving the transition from unicellular to multicellular life. In particular, we predict that the convex curvature of the trade-off selects for specialization and that the curvature shifts from concave to convex as cell-group size increases. We have tested our models in two ways by taking a how and why approach. We have studied the origin of the genetic basis for reproductive altruism in the multicellular Volvox carteri by showing how an altruistic gene may have originated through co-option of a life-history trade-off gene present in a unicellular ancestor. Second, we ask why reproductive altruism and individuality arise only in the larger members of the volvocine group (recognizing that high levels of kinship are present in all volvocine algae groups). Our answer is that the selective pressures leading to reproductive altruism stem from the increasing cost of reproduction with increasing group size which creates a convex curvature of the trade-off function.