COMPLEMENTATION. “The efficiency of purifying (or negative) selection is the product of effective population size and the average mutational fitness effect(s). In large populations, strong selection will quickly purge mutants with lower fitness. Larger population sizes may also result in higher multiplicities of infection, where a given cell supports replication of at least two different genomes. Here, genetic complementation can increase the robustness of the viral population.”
http://www.sciencedirect.com/science/article/pii/S0042682215001580