The oldest arab families are descendants of arabs who came along with the mameluks before the crusades. Most of the palestinian arabs are descendants of laborers who were brought by the ottoman empire to build the africa-asia railway. That is why many families are called "el masri" meaning egyptian, and similar cases.
Maybe there were some jewish families who descended from judea and converted to islam during the conquests, but they were a neglegent minority. Also note that a jewish community remained in the land of israel even after the roman exile, and grew larger at some times during the middle ages. There was a continuos jewish presence there even after the exile.