One of the objections to evolution that I often hear is that “speciation does not occur.” This objection is usually proffered by an anti-evolutionist who is ready to concede that over several generations, an animal or plant can “adapt” but they remain of the same “kind.” This highlights an actual issue in modern biology — there is no uniform definition of what a “species” really is. That makes it very hard to say that one species has evolved into another, because biologists — all of whom will agree that evolution is an observable phenomenon and all of whom agree that natural selection is the best theory available to explain how it happens — cannot quite agree on when speciation actually happens.
When this point in the discussion is reached, the anti-evolutionist will sometimes fold his arms, look smug, and stop, because the impossibility of evolution has just been proven to their complete satisfaction. I’ll leave it to you, Readers, to figure out why that is incorrect; I will show some faith in your ability to apply logic and play “spot the fallacy.” This one is easy enough, made even easier if you take the next step most such arguers take, and say that the inability of biologists to define speciation is proof of the literal truth of the Bible.
The example pointed to is often dog breeding. Sure, the anti-evolutionist will concede, you can breed dogs so that in four or five generations, you’ve made the descendants significantly larger, or smaller, or dominantly of a particular color or whatever other trait you’ve been breeding for. But that’s an adaptation; you could take that same dog who is the result of the selective breeding and cross-breed it with another dog — any dog, really, and offspring that are recongizably dogs will be produced. In fact, you can even cross-breed wolves and dogs* and there are some biologists who will say that they are really the same species.
What I’d like to call your attention to is that speciation appear to take place in observable time. To be sure, the boundaries are fuzzy and indistinct, and not all biologists agree with “speciation” as the divergence of communities of formerly like animals (or plants) such that they can no longer breed with one another. But if that is a definition you can accept, it is something that appears to be happening in the Solomon Islands.
The monarch flycatcher is a small bird that makes its living eating insects. A population of monarch flycatchers immigrated to a set of islands in the chain, such that it is difficult for birds on the large central island to travel to the birds on the smaller, outlying islands, and vice-versa. Over time, the birds on the central island developed all-black feathers; birds on the outlying islands developed a chestnut-colored belly. Then scientists took some of the outer-island birds and put them on the central island, and took some central island birds out to the outlying islands.
This is an experiment out in the wild, so it’s impossible to tell whether the males of one population attempted to breed with the females of the other. But no specimens showing anticipated signs of cross-breeding have been found. And more to the point, the males of the indigenous population do not defend their turf from males of the invader population; they are not considered threats to “breeding rights” the way that other indigenous males are. It is exactly what we should expect to happen if speciation had occurred.
Is this proof of evolution by natural selection? No. If you’re looking for proof of a hypothesis you’re not exactly doing science, since a well-designed experiment is intended to disprove a hypothesis. Rather, this corroborates the theory. It fails to disprove the theory because this is consistent with the theory. What it really does is disprove both the dramatic hypothesis that “speciation has never occurred” or the more modest hypothesis that “speiciation has never been observed.” While experts might disagree as to how conclusive the evidence is, and they might disagree on whether the line has actually been crossed or not, they would all concur that this is a significant movement in that direction. 
* Some people like to keep half-wolves as pets. This is not, in my opinion, a particularly good idea. Half-wolf dogs are difficult to control and train to accept human leadership, and in particular they are prone to bite. Individual specimens may differ, of course, but the dogs you get as pets are the result of millennia of selective breeding to eliminate the very strong defensive traits of wolves in the wild. Sure, wolves can look very pretty but a wolf is a wild animal that can be dangerous; a dog is a domesticated animal that will accept training early in life and more to the point, has smaller adrenal glands than its wild cousins.
