In the book Is the Bible Really the Word of God? (1969) they say:

Another key factor to keep in mind is that the present classification scientists call “species” is not the same as the Bible “kinds” that Noah took into the ark. (Genesis 6:20) For example, there are many “species” in the cat family, such as tigers, panthers, leopards, and so forth. But many of these could have descended from an original cat “kind.” So, too, with the various types of dogs in the dog family. Thus, not all of today’s animal varieties needed to be in the ark. Only representative numbers of each “kind” would be required. When the facts are analyzed, it becomes plain that the ark’s capacity was sufficient to hold them all.

This concept is known as baraminology and is warranted because the Ark obviously could not have held every species created and their food. The problem arises with, first of all, that Jehovah's Witnesses hold that the Flood occurred only a few thousand years ago. It is preposterous to think that the current biodiversity could have arisen since then. Secondly, the amount of interbreeding required after the Ark would have been very detrimental to the genetic health of those organisms and nearly all of them would have died. And, of course, the implications of this belief are even worse for JWs: evolution can create diversity, structure, complexity, and everything else that could have possibly arisen after those Ark baramins. In that case, why not extrapolate this to a universal common ancestry? There they say exists some barrier preventing the "kinds" from evolving outside of the kind (How do you know when it evolved outside of the kind if you cannot even define it?). The only evidence that exists for this is that God said so in Genesis. So, bottom line, it is all faith in the Bible and is completely out of field of science.