SLOWLY DOMINANT MUTANT MORTALS. “Our findings suggest that a mortal organism, born among a population of immortal organisms, cannot reproduce and becomes extinct in many cases. Nonetheless, a number of mortal organisms did manage to survive at a small but significant rate. Moreover, once a mortal organism survives, it extends its habitation area, surpasses immortal organisms and prospers without exception.”
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.657.723&rep=rep1&type=pdf