What breed, alittlelssbutter?

Mammals are more complex, so I can see where more would be able to go wrong.

I bred fruit flies for my dart frogs for 4 years or so from only two original cultures. I got about 48 generations out of each. Never a drop in production, or (that I saw) physical defects like one wing, no eyes, missing legs, etc. However, the flightless flies DID evolve the ability to fly!

Snakes are much less complex than are dogs.

I can only think of it being like a radio flyer vs a harley full dresser. Many more parts on a harley than a red wagon to go wrong.