Two animals are members of the same species only if they can create fertile offspring.
In last week's Parasha, before the time of Noach, the animals were mating with each other and this must have created all sorts of infertile offspring. It would be something if the flood was more than a literal one, where so many infertile animals existed that the real ones had to be protected in the ark.
Sometimes this infertility does not occur until another generation of offspring. Drawing the line is not simple when two species are close but not the same. To an extend, racial division among people constitute a mild speciation of man since different groups have different gene pools. This is why a prosperous society can be handed over to a bunch of savages and the savages can almost instantaneously destroy it. Of course people from different gene pools can also mate to fill in missing genes to combat genetic defects.