Usually the dog is said to form the species <I>Canis familiaris</I>, but genetically dogs belong to the same species as the wolf, <I>Canis lupus</I>.
However, don't overuse the I element. In particular, for emphasis use EM or STRONG, and for variables (placeholders) use VAR. See general notes on text markup.
Words and phrases taken as such from other languages (than the language in which the document is written), such as status quo, Weltanschauung or sauna, are often presented in italics. However, the more common the word or phrase is (in your text or in your language in general), the less the reader benefits from designating them as foreign and the more he may be disturbed by the frequent occurrence of different fonts in the text.
In linguistics, when referring to words and phrases as in "the plural of ox is oxen", it is normal to use italics. (HTML 2.0 suggests the use of SAMP for such purposes, but that would be unnatural.)
The rules for scientific names for organisms say that the names should be written in italics if possible, so it is natural to write them within I elements. The same applies to symbols of physical quantities such as F for force; the VAR element might sound suitable, but I elements are rendered in the required way, in italics, more probably than VAR elements are.