Final answer:
Biogeography is the field of biology that uses the biogeographical distribution of species and ecosystems to understand their relatedness.
Step-by-step explanation:
The field of biology that primarily falls under the approach of using the biogeographical distribution of species and ecosystems to understand their relatedness is Biogeography. Biogeography is the study of the geographic distribution of living things, both past and present, and how the environment affects species. It is critical in understanding our physical environment and has played a significant role in developing modern evolutionary theory.