 If, in contrast, migration occurs by means of large populations, higher heritability of traits and a more representative sampling of the parent population will result. Each propagate is made up of individuals derived from a single population, and there is no mixing of colonists from the different populations during propagule formation. On the basis of further analysis, much more between-population genetic variance can be maintained with the propagule model (Slatkin and Wade 1978: 3531). Thus, by using propagule pools as the assumption about colonization, one can greatly expand the set of parameter values for which group selection can be effective (Slatkin and Wade 1978, cf. Craig 1982).