Indirect genetic effects should make group size highly evolvable

Group size is an important trait is many ecological and evolutionary processes. However, it is not a trait possessed by individuals but by social groups, and as many genomes contribute to group size understanding its genetic underpinnings and so predicting its evolution is a conceptual challenge. Here I present a suggestion for how group size can be modelled as a joint phenotype of multiple individuals, and so how models for evolution accounting for indirect genetic effects are essential for understanding the genetic variance of group size. This approach makes it clear that 1) group size should have a large genetic variance as indirect effects always contribute exactly as much as direct genetic effects and 2) the response to selection of group size should be rapid as the correlation between direct and indirect effects is always at the maximum positive limit of 1. Group size should therefore show rapid evolved increases and decrease, the consequences of which and evidence for I discuss.