SELECTION FOR GROUP. “There is direct experimental evidence for higher-level benefits associated with POD in unicells. In bacterial species, several studies have compared group-level function in POD-positive versus POD- negative strains.” (POD = programmed organismal death).
http://www.esciencecentral.org/journals/are-internal-death-promoting-mechanisms-ever-adaptive-2329-9002.1000113.pdf