Molecular Underpinnings of Speciation

You discover two new species of newt. One newt is purple and has four legs, while the other is orange and has six legs. You accidentally keep them in the same cage, and the next day, your purple newt is pregnant! It gives birth to a purple-orange newt with five legs. You find out that a colleague of yours experienced the same thing, and he brings over his purple-orange five-legged newt. Before you know it, the two purple-orange five-legged newts scurry off and yours gets pregnant. When it gives birth, however, the baby newt does not live for very long. Any further attempts for the purple-orange five-legged newts to have children end in the same way. What type of isolating mechanism is this?