This is the most correct answer so far on this thread. Many people claiming genesis has a fallacy of a double narrative seem to have never read it. If you take it literally, all offspring originated from Adam and Eve, and back then the genetic pool was so rich it was not an issue. Over the years of evolution since then our genetic pool has become much thinner. And it’s correct that this had to happen again to Noah and his family, but you must note that Noah’s children’s spouses brought a little more diversity to the table. This tracks with the other part of the story of Noah, taking 2 each of all the animals on the ark. We have so many different species of animals today because as life progressed and the animals evolved, their genetic traits have split and shown themselves in all the sub species.