This determines whether the association will be open (indetermined number of monomers) or closed (fixed number of monomers).
Angel Mozo-Villarías, Juan Cedano, Enrique Querol 2014, 'A model of protein association based on their hydrophobic and electric interactions.', PLoS ONE
A subpopulation includes a fixed number of candidates rather than a fixed radius.
Magda B. Fayek, Nevin M. Darwish, Mayada M. Ali 2010, 'Context based clearing procedure: A niching method for genetic algorithms', Journal of Advanced Research