In the Shabbos shacharis birkos kerias shema, there is a poem referred to as Kel Adon that we say.
In some congregations, the custom is to say the stanzas responsively, with the chazzan saying each stanza and then the congregation repeating it. In other congregations, the custom is that everyone says the entire poem straight through together.
I am (under the assumption that one custom was the original way and the other evolved later) curious when whichever custom is later started. All sourced answers are welcome.