[susan53]

It depends what type of relative clause it is - defining or non-defining.

- A defining relative clause tells you which person/thing etc I'm talking about, and no comma is used : I have two brothers and a sister. One of my brothers lives in France and one in the States. The one who lives in France is a famous neurosurgeon. - the relative clause specifies which of the two brothers the new information refers to.

- A non-defining relative clause just gives you an extra piece of information about the person/thing (etc) : I have a brother and a sister. My brother, who is a famous neurosurgeon, lives in the States. here, commas are used.

Your example is almost certainly non-defining - the speaker is just giving us information about the deer - and so the comma could be used. Compare :

Synthetic musk oil would help conserve a certain species of deer, whose gland is the source of musk. (Non-defining)
Synthetic musk oil would help conserve the species of deer whose gland is the source of musk, but would do nothing to help the other species at risk. (Defining)