Well, besides the usual book evidence given for this theory, Tywin now describes how he wanted to kill Tyrion the moment he was born.  But, if he did not have reason to believe that he was someone else's child, he would not have known about Tyrion's physical traits so soon after birth, meaning he had other reasons for…

I think this show made it more clear that Tyrion is not a Lannister, but is actually a Targaryan as the theory goes.  Tywin all but admitted that Tyrion was fathered by another individual, namely Aerys Targaryan..