I have a serious question plz help?

ok my last name is davis but my father comes from dominican republic and my mother from puerto rico and my father parents is from d.r. and their parents is from d.r. and my mother parents is from p.r. and my grandparents parents is from p.r. how is this and they have last names davis and wilkerson please tell me this im confused and do all spanish people have spanish last names like gomez or sanchez etc