Because nature does not contradict it's self and nature is demonstrating that a biodiversity species biologically reproduce the same biological reproductive species and to say other wise would be in contradiction on what nature demonstrates.<quoted text>What makes you think bio-diversity is "fixed?" The evolutionary process continues, nothing about it is "fixed."
Your evolutionary process is advance ET's or an advance terrestrial being(s) genetically engineering one or more different biological reproductive species to create a new biological reproductive species. For example, the genetic engineers could have use some of the great apes or monkey genetic material to create humans. This is what you are observing and calling evolution.