By hybridization, if at all. Here:
Interbreeding between animal species usually leads to offspring less vigorous than either parent—if they survive at all. But the combination of wolf, coyote and dog DNA that resulted from this reproductive necessity generated an exception. The consequence has been booming numbers of an extraordinarily fit new animal (see picture) spreading through the eastern part of North America. Some call this creature the eastern coyote. Others, though, have dubbed it the “coywolf”. Whatever name it goes by, Roland Kays of North Carolina State University, in Raleigh, reckons it now numbers in the millions.
This has been going on for thousands of years. The different groups, wolves, dogs, and coyotes, each have differing advantages that usually keeps them distinct, but not always.
The mixing of genes that has created the coywolf has been more rapid, pervasive and transformational than many once thought.
…
Whether the coywolf actually has evolved into a distinct species is debated. Jonathan Way, who works in Massachusetts for the National Park Service, claims in a forthcoming paper that it has. He thinks its morphological and genetic divergence from its ancestors is sufficient to qualify. But many disagree. One common definition of a species is a population that will not interbreed with outsiders. Since coywolves continue to mate with dogs and wolves, the argument goes, they are therefore not a species. But, given the way coywolves came into existence, that definition would mean wolves and coyotes should not be considered different species either—and that does not even begin to address whether domestic dogs are a species, or just an aberrant form of wolf. More.
Domestic dogs are not a species; they are an aberrant form of wolf that learned to live with humans as the lowest members of a pack order, in exchange for food and protection. Other, probably bigger and fiercer, wolves preferred their independence.
The problem is, once this is admitted, lots of Darwinblather becomes obsolete.
See more on hybridization as a form of evolution at: Life continues to ignore what evolution experts say
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