Different genera within a family share the same basic body plan. Clearly, as one ascends the hierarchy, similarities among the units of classification become less and less. The classification hierarchy. Common names differ from place to place, but scientific names provide worldwide consistency. Note the convention that genus and species are italicized, and the species name begins with a lower-case letter.
Postal address is another, familiar classification hierarchy.
Today, the classification hierarchy makes biological sense because it reflects the relatedness among the living descendants of past evolutionary branchings in the tree of life . Perhaps surprisingly, because the kingdom is the highest level of organization, the number of kingdoms of life is uncertain. This might make more sense when we think that to define a kingdom one must reach back to the very origins of life, and attempt to locate the most fundamental branchings. Whether fungi evolved from photosynthetic green algae, losing their capacity to photosynthesize, or evolved from a non-photosynthetic precursor, also is uncertain. Further changes affecting bacteria and protozoans are almost certain to occur.