How Did God Separate Gentile From Jew?

If humans all came from Adam & Eve, and then later from the generations after Noah, then it seems that we are all very distantly related. So, when God spoke to Abraham and created the covenant, and Abraham's descendants became the chosen people, how did God distinguish them from Gentiles? What was the difference? How were Abraham's descendants no longer related to the Gentiles? What separated Gentile from Jew if they descended from a common ancestor? Thanks for your thoughts.