Social systems induce extreme contrasts on evolutionary speed in silico

Sociality has been suggested as a source of differences in evolvability between species, but predictions are lacking concerning the effects of different social dimensions. Using computer simulations, we provide a grain of salt in this direction. We varied four parameters along four social dimensions: Organization - 1) which sex disperses and 2) the number of males and females; Structure and Mating systems - 3) the levels of reproductive skew among males and females; and Care systems - 4) the presence or absence of cooperative care. This social variation was simulated according to the genetic transmission of some of the major taxa studied, haplodiploidy and diplodiploidy to emulate mammals, birds and Hymenoptera. To measure the impact on anagenesis, we assessed the spread of a hyper-successful variant in an alpha individual after three generations. We hypothesized that the higher this capacity for spread, the faster the speed of change in a species. To investigate cladogenetic effects, we measured the number of genetic lines that reproduced and the equitability of their contribution as a measure of how much mixing induced different social systems. We hypothesized that the more diversity the less chances of speciation to ensue.

Variant spread and genetic diversity were strongly negatively correlated but showed their highest variation at different sites. Spread varied most at high group sizes while diversity changed most at the low end of the spectrum. Anagenesis and cladogenesis were the result of complex interactions between our social variables, endorsing recent claims for the need to measure social systems along multiple dimensions. We found that the number of breeding females in a unit and the level of reproductive skew determined variant spread through the number of dispersing offspring produced. Moreover, group augmentation increased potential spread because under constant population size fewer units existed. Hyper-successful variants in emigrants could flood the breeding positions of a smaller effective breeding population size and thus contribute disproportionately to the next generation. The social systems with highest variant spread were characterized by male biased dispersal and monopolization on big multi-male-multi-female groups and female- or male-biased dispersal with cooperative breeding where the alpha female could access the units’ full reproductive potential through cooperative care. Genetic diversity was mostly affected by the number of immigrants in units and the total number of units. The more group sizes increased through philopatry, the fewer number of distinct philopatric lines in different units there were, decreasing diversity. The more groups increased through immigration, the higher the inequality. Because philopatric lines always gathered at least 50% of genetic contributions, immigrants shared the rest. If there were more immigrants, each of them gathered less. The social system inducing the highest genetic diversity was characterized by a population of monogamous pairs while the lowest had highest multi-male-multifemale groups with only a monogamous pair reproducing. Overall, our model supports previous frameworks and empirical studies linking stronger sexual and social selection with faster adaptive and neutral anagenesis but is less suited to test for cladogenesis. We propose some evolutionary speed scenarios according to previously reconstructed social transitions in different taxa.