While species are indeed a fundamental unit in classifying organisms, they are not the lowest subgroup. There are smaller, more specific classifications below species:

* Subspecies: Groups within a species that have distinct characteristics and are geographically isolated.

* Variety: A group within a species that has a specific trait, often a morphological difference.

* Form: An informal designation for a group within a species that has a specific characteristic, often related to behavior or ecology.

Therefore, the lowest subgroup for classifying organisms depends on the specific system used and the level of detail desired.