The largest group that is capable of interbreeding is

i believe the answer is pupils are unequal and non-reactive

the pupil of both eyes has different size, so you could say the pupil is unequal. pupil has muscle that makes it able to constrict itself according to the light intensity. when the doctor direct their flashlight to the eye, pupil should be constrict to reduce the light intensity. if the pupil doesn't change their diameter, it is called unresponsive.