Your argument about speciation is merely your ignorance. Firstly, some closely related species do have an enormous spectrum, as you describe about dogs. It may be worth noting that some breeds of dogs could be classified as different species, since they cannot interbreed anymore. That's an aside. The main point is that your expectations are way off. When a populatiin lives together, they interbreed, and therefore exchange genes, and traits continue within the whole population. A branch occurs when a population is split, and the two populations can no longer interbreed. Then you should expect the populations to diverge, and this is exactly what happens. Note, this has actually been tested.