A taxonomic family is a taxonomic rank above a genus and below a taxonomic order. A particular family is a named group of organisms placed at that rank; the rank itself is a position in the classification hierarchy.[ref]
For example, the cat family Felidae includes the genera Felis and Panthera. The family groups several genera, while each genus groups species. Scientific classifications draw on morphological, molecular and other evidence when deciding which organisms belong together.[ref]
Classification of living cat species by the IUCN SSC Cat Specialist Group. Cat Classification Task Force account and evidence underlying the 2017 revision
A constructed counting example separates hierarchy from abundance. If a hypothetical family contains two genera with three and five recognized species respectively, it contains eight species, provided those lists are disjoint and complete. It need not contain eight individual animals: the same eight species could be represented by millions of organisms.
The rank does not prescribe a fixed number of genera or species. Nor is a family label itself a measurement of evolutionary time. A phylogenetic tree represents hypotheses about ancestry and branching; ranked names organize those organisms at selected hierarchical levels. Changes in evidence can lead to revised membership without changing what the rank word means.[ref]
Biology 2e by Mary Ann Clark and others. section 20.1, phylogenetic hypotheses and classification revisions
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