Why can life span be difficult to define in bacteria, algae, and other organisms that reproduce by division?

Defining life span is difficult for organisms that reproduce by binary fission, such as bacteria, algae, and many protists, because the concept of an "individual" is not as clearly defined as it is in sexually reproducing species.1

In sexual reproduction, an individual has a distinct beginning—often marked by the formation of a fertilized egg—and a unique genetic identity. In contrast, organisms that reproduce by division continually split into genetically identical daughter cells. Because these organisms are constantly dividing, their "individual" existence must be defined arbitrarily.12

Furthermore, these organisms often lack the clear aging processes associated with individuality. While sexual organisms possess a life span determined by heredity and internal aging, many colonial or dividing organisms appear to lack these aging processes and may be capable of surviving indefinitely unless ended by external factors. Consequently, because the parent organism essentially continues to exist in its offspring, the life spans of these organisms are not strictly comparable to those of sexually produced forms.13