General Discussion

basically every ant in a colony is the same organism - the result of an eternally-cloned fertilized egg from a single queen. it might be more accurate to regard individual ants as organs of the colony rather than individual creatures, because really, that's how an ant colony works.

In the case of the ant colony you describe - this one? - my guess would be at some point the original queen's egg-replication went a little faulty - maybe due to being exposed to toxins, a virus, or some other mutagen - and resulted in an "improper copy" - which led to a mutant strain of (apparently fertile) ants that the rest of the colony would still recognize as "one of us' rather than "invasive intruders." The species has since continued its own parasitic evolutionary course.