SYMMETRY EXCEPTION. “An asymmetrically dividing bacterium has recently been found to show senescence. Remarkably, however, even the symmetrically dividing E. coli ages: it shows subcellular mother-offspring asymmetry, delineating age classes upon which selection can act to produce senescence.”
http://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151
