I believe this should answer your question adequately. The short version is that the patrician order consisted of those families that claimed direct descent from Romulus' original patres, as well as those whose families had been Italian nobility inducted into the order later. Inclusion in the order consisted entirely of birth and nothing else, and by the Punic Wars the order was already dying out. By the late Republic they were, for all intents and purposes, extinct, and the senatorial class had for centuries consisted mainly of plebeian nobiles