Genus species

When writing the genus and species of an organism, the genus name is capitalized, but the species name is not. Also note that both are usually italicized to differentiate them from common names.

Right: Homo sapiens
Wrong: Homo Sapiens, homo sapiens, Homo sapiens

This is a long established rule that is consistently followed by scientists but generally ignored by many science reporters and other non-scientists. It becomes very useful when taxonomic level could be unclear.

For example, the extant members of Bison are bison (the American bison) and bonasus (the European bison, or wisent). While bison has been out of immediate risk of extinction for some decades, bonasus populations remain small and have started to rise only recently in captivity and intensely managed reserves. Commercially available bison meat is bison. It is more commonly sold as buffalo meat, although the term buffalo is more properly used for Syncerus and Bubalus, rather than Bison.

It is prefered, but not in all contexts, that in scientific writing one put the abbriviated genus name ahead of the species name. This is done in part because many genera have identically named species. For example, Dendrocopos major, Parus caeruleus and Parus major are all birds of Europe. While P. major is clearly in Parus, P. caeruleus is sometimes placed in Cyanistes.