How are new species formed?
Stage 3: Two Different Species.
Take the case of domestic dogs. It is recognized that our dog
breeds probably result from the domestication of wild wolves thousand of years
ago by early cave people. All dogs have largely the same DNA, and their body
form, although occasionally drastically altered in some breeds, is still largely
the same. One dog breed can still interbreed with other dog breeds to produce
viable, reproducing offspring.
(Note that dogs can also still interbreed with wolves and coyotes
to produce viable, reproducing offspring. We consider dogs separate species
from wolves and coyotes because the domestic and wild dog breeds rarely would
have a chance to get together and interbreed.)
Long & Slichter