Praeluceo

So if adaptation is clearly happening, and genetic mutation is happening too, is it not possible that 2 animals which we currently have as different species are actually the same and have adapted enough and mutated enough under different environments to where they no longer produce fertile offspring? Is there perhaps a genetic mutation distance that is quantifiable that prevents the creation of fertile offspring between what could've been 2 different adaptations of the same species?

So in the case of lions and tigers, that can breed and make "liger" offspring, they are still differentiated by the fact the offspring are fertile then? And to the contrary, if an offspring was fertile they would be considered the same species?

Someone posed this question to me and it has kind of been bothering me because I cannot find a sufficient answer. The basic question is what standard do we use to classify animals? For instance, if you have 2 birds that look very similar and have similar eating habits, but different coloring, are different sizes, what makes us decide it is a different species or not? That is the first question. The second question is if mankind are currently all considered homo sapien, why do we not apply the same tools to classify humans as we do animals? (this is based on the understanding that appearence, habits, diet, etc are things factored into establishing a classification of an animal or new species). The third and last part of this questions is, could or has science come up with a genetic or DNA method of differentiating between animal species and if so is it universally applied to homo sapiens as well? I realize some of this could be pretty controversial and what not, but I have not located a sufficient answer. Thanks for anyone that could point me to some good source material to review.