Phylogenetic classification groups, called taxa (plural) or taxon:

Phylum/phyla (called division/divisions in botany)

Class/classes

Order/orders

Family/families

Genus/genera

Species/species

Each group or division or taxon is made up of a set of organisms having a certain set of fundamental morphological similarities not possessed by other organisms. This grouping of organisms should represent a natural classification system as opposed to an artificial one set up using superficial characteristics.

This phylogenetic sequence indicates the age of the characteristic structures with the species grouping being of the most recent origin and the phylum being the most recent. In other words, the characteristics that differentiate species came into being fairly recently, whereas those that differentiate phyla came into being a long, long time ago.

The genus is a group of closely related species (and is thus comparable more or less to a surname or family name) and is given first and capitalized whereas the species is a group of actually or potentially interbreeding individuals or populations in which no breeding occurs with other groups. The species name is given second, not capitalized. the scientific name is considered special and is thus italicized in print or underlined when typed or handwritten:

Canis familaris, or the domesticated dog; Canis aureus, or the jackal; Canis lupus, or, the wolf.

Panulirus interruptus, or the California spiny lobster; Panulirus inflatus, or the Mexican langosta; Panulirus argus, or the Caribbean or Florida spiny lobster. The Maine lobster is not a spiny lobster; thus. it is not in the same genus, and, in fact, is not even in the same family as the spiny lobsters. its scientific name is Homarus americanus.