 This simple model assumes that migration is the only factor affecting gene frequency at the locus, but this is unlikely to be the case. So it is necessary to consider how migration will interact with selection, drift and mutation (cf. Rice 2004, ch.5). A balance between migration and selection can lead to the maintenance of a deleterious allele in a population, in a manner analogous to mutation-selection balance. The interaction between migration and drift is especially interesting. Genetic drift will often lead the separate subpopulations of a species to diverge genetically. Migration opposes this trend—it is a homogenising force that tends to make subpopulations more alike. Mathematical models suggest that that even a fairly small rate of migration will be sufficient to prevent the subpopulations of a species from diverging genetically (Hartl and Clark 2006). Some theorists have used this to argue against the evolutionary importance of group selection, on the grounds that genetic differences between groups, which are essential for group selection to operate, are unlikely to persist in the face of migration.