Question

I am confused about the punctuation in this sentence: It is not a pretty picture, and some express doubt that such people exist.
I'm confused as to why the comma is placed where it is. Is " It is not a pretty picture," a dependent clause? Is "and some express doubt that such people exist," a dependent clause?
I'm unsure of which comma rule this applies to.
Am I linking an independent clause to another independent clause with a coordinating conjunction? Are these two sentences even independent?