Generation time and fitness tradeoffs during the evolution of multicellularity

The evolution of multicellular organisms from their unicellular ancestors is an example of an evolutionary transition in individuality (ETI), i.e. a change in the units of selection and adaptation. The theory of ETIs poses particular challenges because, by definition, key theoretical constructs such as fitness are shifting during an ETI. Past work emphasized the importance of life history tradeoffs during ETIs in which lower level units form groups and become individuals at a higher level. In particular, it has been hypothesized that the convexity of the lower-level tradeoff between viability and fecundity changes with group size and determines the optimality of lower-level specialization in the fitness components of the group. This is important because lower-level specialization is a key indicator of higher-level individuality. Here we show that increasing generation time can increase the convexity of the lower-level viability-fecundity tradeoff. This effect is a novel hypothesis for the positive association between cell-group size and cellular specialization in a major model system for ETIs, the volvocine algae. The pattern in this clade is thought to be an example of a more general size-complexity rule. Our hypothesis is that larger groups have longer generation times and longer generation times lead to more convex lower-level viability-fecundity tradeoffs, which could account for specialization being optimal only in larger cell groups (colonies). We discuss the robustness of this effect to various changes in the assumptions of our model. Our work is important for the study of ETIs in general because viability and fecundity are fundamental components of fitness in all systems and because generation time is expected to be changing during many ETIs.