yes there is, and modern genetics has proven it. you can go do any number of studies regarding, say, the breeding of cows, and there ALWAYS a point to which the breed can no longer be modified. the Clydesdale horse is a prime example. it's progeny aren't Clydesdales. they either a Belgian Draft horse, or whatever the other parent is (i forget.)
a horse is a horse is a horse. a horse will never NOT be a horse. a horse will never have babies that are not horses.
Siamese cats. same thing. babies are either Siamese or all black. no other choices.
you don't understand how speciation works. when a new species is formed, the result is more focused, and has less inherent diversity. the flow is in the opposite direction.
there are a million dog breeds. which breed isn't a dog anymore? for that matter, which one is closest to not being a dog any more. go ahead. you're so smart. tell us.