When classifying organisms using a dichotomous key which category is the narrowest or will contain the least number of organisms

Species

Explanation:

The highest taxonomic category is domain followed by kingdom, phylum, class, order, family, genus and species. Among this, being the highest category, domain includes maximum number of organisms. On the other hand, being the lowest taxonomic category, species would include least number of organisms.  

Example: Domain “eukarya” includes all the eukaryotic organisms while species "sapiens" includes only present day human beings.

Life is classified in many levels from less specific to more specific:

Domain (bacteria, archaea, eukaryotes)

Kingdom

Phylum

Class

Order

Family

Genus

Species

The domains 'contains' the most number of organisms, species contain the least number of organisms.

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