By selecting the best-tasting or highest-yielding beans, farmers inadvertently reduced the genetic diversity. This doesn't just apply to beans it applies to all crop species. As an example, let's think about aliens to coming to Earth and abducting the population of a small town to populate a new planet. Any small town in the world cannot represent the genetic diversity of our entire planet! So, you can see the new planet would suffer from reduced genetic diversity. That's what has happened with crop diversity during domestication.