Biblical ignorance on parade.
First, there was no concept of “species” in the days of Noah. There were “kinds” — which is different from “species”. I suppose we could think of “kinds” as more like a “genus” than a “species” ... but regardless, since the days of Noah, the various species of creatures has expanded, not as evolution proposes, but as creation science proposes. See the Orchard model discussed here: http://creation.com/refuting-evolution-chapter-2-variation-and-natural-selection-versus-evolution
Second, there were not simply two of every land animal onboard the ark. There were *seven* of every kind of clean land animal onboard.
Though the concept of species was not around at the time, they would have to still be there.