Some aspects of reproductive mode and the origin of multicellularity. Selection 1:97–109

Evolutionary transitions require the organization of genetic variation at two (or more) levels of selection so that fit-ness heritability may emerge at the new higher level. For example, in case of the transition from single cells to multicellular organisms, single cells must, as it were, relinquish their claim to flourish and multiply in favor of the multicellular group. In this paper we consider the consequences on fitness variation and heritability of two main modes of reproduction used in multicellular organisms: vegetative reproduction, where the offspring originates from a group of cells of the adult (a propagule), and single-cell reproduction, where development starts from only one cell. Most modern organisms pass through a single-cell stage during their life-cycle, a possible explanation being that the single-cell stage increases the effectiveness of organism selection relative to cell selection, by increasing the kinship among cells within the organism. To study this hypothesis we consider simple cell colonies reproducing by frag-ments or propagules of differing size, with mutations occurring during colony growth. Mutations are deleterious at the colony level, but can be advantageous or deleterious at the cell level (termed selfish or uniformly deleterious mutants, respectively). In our model fragment size affects fitness in two ways, through a direct effect on group size (which in turn affects fitness) and by affecting the within and between group variances and opportunity for selection on mutations at the two levels. We show that the evolution of fragment size is determined primarily by its direct ef-fects on group size, except when mutations are selfish. When mutations are selfish, smaller propagule size may b