Answer:
That is why good breeders import new lines...
I am sick of the rumor that all purebreds are "inbred"
ADD -
http://www.pedigreedatabase.com knock yourself out and stop believing silly rumours started by backyard breeders.
There are hundreds of breeds registered with the AKC and they do occasionally add new one but it takes decades of careful breeding to get a dog recognised as it takes that long to develop a breed standard.
ADD - I am confused now... are you saying that a different breed should be introduced into a current breed? (say, introducing a mastiff into a great danes line)
Or that just a different line be introduced (say you imported a dog from another country of the same breed)
The first one... heck no! that would make it a cross bred not a purebred.
The second one, yes definately! introducing a new line every so often is great.
ADD - Ahh ok, yes I understand what you are saying.
In theory yes it sounds like a good idea but then in comes the dominant and recessive genes into play...
The new dog that is introduced may have sets of dominant genes that would overpower the current dog completely changing its appearance, even if the dog being introduced was 75%...
What I am saying is that with just introducing a new line that is also purebred this can improve the breed as well.
I will use my breed for example... Poodles
Scandinavian imports are very highly sort after by many poodle breeders as they havea different 'type' than what we have here. They have everything that the breed standard calls for the same as ours but they are different... I find Scandinavians to look more impressive as they tend to have a better coat, and are the boys are very masculine.
My dogs are more refined but the males are still obviously male...
If I mix the two I would (in theory) get somewhere in between the two changing the appearance of my dogs but still maintaining the breed standard.
I think I have gone a bit side tracked here but I hope I put my point in there somewhere!