[shuckmaster]

Would giving birth to a different species contradict it?

Any fertile offspring is always the same species as the parents.

[tallhappy]

Any fertile offspring is always the same species as the parents.

So how do new species arise?

[Doctor Stochastic]

Not always. Instant tetraploidy (or polyploidy in general) can give rise to non-interfertile but clearly intrafertile (even with different parents) offspring. This happens with plants more often than with animals though.