Why? I can imagine discovering disqualifying defects that falsify the theory of biological evolution by natural selection. In this instance, I don't know of any disqualifying defects and don't believe that they exist. I'm just arguing that it isn't outlandish to think that Darwin's explanation is at least potentially falsifiable. Popperians would insist on it. Falsification of theory A is in no way dependent on whether we are able to propose a better theory B. Those are two separate issues.