'Social heterosis' as a process that maintains genetic variation--a comment.

In the 'mating pool' mode of reproduction, offspring genotypes at each generation are taken randomly from a common population and subdivided into groups where individuals representing a finite sample from the pooled distribution reproduce proportional to their fitness. Assuming that genetically diverse groups contribute more offspring, a recent article by Nonacs & Kapheim [J. Evol. Biol. 20 (2007) 2253] shows that allelic diversity can be easily maintained and proposes the process of 'social heterosis' as a potentially powerful mechanism that accounts for a significant amount of genetic variation. Contrary to their suggestions, I show here that there is a reduced efficiency of selection and an increased probability of fixation of segregating alleles when many loci are simultaneously undergoing social heterosis even if independence of action of the different loci and linkage equilibrium are assumed. The critical issue is that linkage disequilibria are generated by the sampling process of creating small groups and interfere with selection.