Naming an organism by genus and species alone works because those two ranks are:

1. The classification hierarchy runs from the broadest rank down to the narrowest. 2. Domain sits at the top, and within each domain is a second category called a kingdom. 3. Genus and species sit at the bottom of that nesting. 4. Scientists generally refer to an organism only by its genus and species, in what is called binomial nomenclature. 5. Naming by the broadest ranks would fail to identify the organism at all. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 20 'Phylogenies and the History of Life', sections 20.1-20.3_