EMPIRICAL INSIGHTS INTO MULTILEVEL SELECTION THROUGH TRANSITIONS FROM SOLITARY TO COLONIAL ORGANISMS

Major transitions in evolution occur when a new hierarchical level of organization is incorporated into organisms. When this occurs, the dominant level of selection shifts to the highest level of organization in two phases. The first is the emergence of a new level through the aggregation of organisms, followed by the individuation of that new level. The two classes of models of multilevel selection roughly correspond to these phases. The set of multilevel selection models are heterogeneous enough that distilling the biologically important insight is difficult. However, a number of workers have suggested that propagule size is critical for the efficacy of the higher level of selection over the lower level of selection. They note that a small propagule size works because it maximizes the heritability of group-level adaptations. Also note that small propagule size also effectively shifts the time-scale of selection from that of the constituents to that of the group because group reproduction is then contingent on the successful survival of that propagule. I suggest that the propagule size parameter is best measured as the proportion of the group that does not reproduce, the non-reproductive ratio (NRR). Colonial organisms provide almost continuous variation across the full range of possible NRRs, many primitive groups with a NRR of zero and many with NRRs approaching one. Preliminary data show that colonial organisms with low NRR are weakly individuated and that a positive correlation between NRR and individuation is present across 189 taxonomic groups within 13 phyla and 3 kingdoms.