Cladistics rests on a distinction of “ancestors” and “descendants”. This distinction is, however, problematic. The problem is that it implies that every descendant also is an ancestor, and thus also that every ancestor is a descendant, which actually contradicts the distinction itself. (Ie, if every descendant also is an ancestor, and every ancestor also is a descendant, then the distinction distinguishes nothing)
It means that cladistics is fundamentally inconsistent. And, since the foundation for cladistics also is the foundation for race biology, that also race biology is fundamentally inconsistent.
It is thus fairly easy to show that race biology (and thus also cladistics) is fundamentally inconsistent (luckily).