The paragraph suggests that the two cells resulting from cell division are not likely to be absolutely identical. The reason for this is explained by the inherent imperfections in the biochemical reactions involved in DNA copying during reproduction. No biochemical reaction is deemed absolutely reliable, and, as a consequence, variations can occur during the replication of DNA.

These variations may be subtle, but they contribute to the genetic diversity among the resulting cells. While the two cells share a common origin, the potential for small errors or mutations during DNA copying means that the copies may not be exact replicas of each other. As a result, the surviving cells are described as “similar, but subtly different from each other.” This built-in tendency for variation during reproduction is recognized as the basis for evolution.