The Species problem is a dilemma facing Biologists (Taxanomists and Phylogeneticists to be more specific). This dilemma is what it takes for two species of the same Genus from one another.
Now one would say "obviously its that's they can't produce fertile offspring"
But that's the dilemma. That requirement is steadily becoming outdated. As new evidence shows that Neanderthals and Homo Sapiens regularly interbred with each other. And new breeds of canine dubbed Coywolves (coyote and wolf hybrids) have been shown popping up in the wild, and they are fertile.
So how exactly could we solve this problem?