mixed bag vs selection

mixed bag and selection both are nouns.

Word	Noun	Adjective	Verb	Adverb
mixed bag	Yes	No	No	No
selection	Yes	No	No	No

As nouns, selection is a hyponym of mixed bag; that is, selection is a word with a more specific, narrower meaning than mixed bag:

mixed bag (noun)	selection (noun)
a collection containing a variety of sorts of things	the act of choosing or selecting
	the person or thing chosen or selected
	a passage selected from a larger work
	an assortment of things from which a choice can be made
	a natural process resulting in the evolution of organisms best adapted to the environment