How do I do MLA citations for an anniversary edition, e.g. 30th anniversary Selfish Gene. Do I treat it as a normal edition or just ignore that it is a new edition?
The following is quoted from Harvard Guide to Using Sources
When you are citing an edition of a book other than the first edition, you should indicate the edition. In both MLA and APA styles, you should identify the edition you are citing by year or number (if either is available), or by name (if the edition is listed as "revised" or "abridged").
Therefore, when citing an anniversary edition, you would simply mention the name of the edition.
For example, (borrowed from here; The link is of a cached version; search for "anniversary" to find this entry)
Hyman, T. S. (1974). Snow White. (Silver Anniversary Edition). Boston: Little, Brown.
Hope this solves your problem.
You would need to cite the fact that it's a new edition, purely because the page numbers might be different to the old edition. So that your reader can find the exact citation using your reference, you should always be as specific about which edition you're using as you can be.
From the OWL at Purdue (my go-to site for MLA):
A Subsequent Edition
Cite the book as you normally would, but add the number of the edition after the title.
Crowley, Sharon, and Debra Hawhee. Ancient Rhetorics for Contemporary Students. 3rd ed. New York: Pearson/Longman, 2004. Print.
The entire 'Works Cited: Books' guide is here: https://owl.english.purdue.edu/owl/resource/747/06/
So in your case, the reference would look something like this:
Dawkins, Richard. The Selfish Gene. 30th Anniversary ed. Oxford: OUP, 2006.