Theistic Evolution, the hypothesis that God created all life through evolutionary processes, has been criticized by some atheists as being "ad hoc". An ad hoc explanation is an explanation conjured up out of thin air in an attempt to keep a belief or hypothesis from being falsified. Why do they say that TE is ad-hoc? Because they view evolution as being a theory that disproves the existence of God by rendering him unnecessary to explain the origin of animal life. If evolution renders the hand of God superfluous, as atheists contend, then positing Him as being an undetectable guide of evolutionary processes is an attempt to keep theism afloat.
I don't think Theistic Evolution is ad-hoc at all. As I pointed out in a recent blog post, I can give the atheists evolution for free and the case for Christianity still comes through. This is because there a various arguments for God's existence (and even for creation and intelligent design) that don't depend on evolution to be false in order for their conclusions to be reached. For example, there's The Kalam Cosmological Argument which argues for the existence of a transcendent Creator on the basis of the origin of the universe and the law of causality, there's The Fine Tuning Argument which argues for an Intelligent Designer of the constants and quantities of nature (e.g The Strong and Weak Nuclear Forces, Gravity, Electromagnetism, etc.) so that the universe can be life permitting, there's The Local Fine Tuning Argument which argues for an Intelligent Designer of the hundreds of characteristics that must be just right in order for a particular region of the universe to be life permitting. There's also The Moral Argument which argues for the existence of God as the grounds for objective moral truths, and there's The Ontological Argument which argues that there mere modal possibility of the existence of God entails that God must actually exist. Moreover, you have The Transcendental Argument which argues that if we weren't created by God, then our reasoning faculties would be unreliable, but sense we can trust our reasoning, it follows by modus tollens reasoning that our minds must have been fashioned by a Creator. You also have The Argument From Desire. On top of all of these natural theology arguments, you have arguments from fulfilled prophesy, and you have the historical case for the resurrection of Jesus.
Now, suppose you're a person who is convinced that all of the arguments I just referenced and linked to are successful arguments. But let's suppose that you're also convinced of the scientific arguments in favor of Darwinian macro evolution. Suppose you think that all of the arguments mentioned in the previous paragraphs are good arguments, but you also believe the scientific arguments for Darwinian macro evolution. What position would it make sense for you to take? The obvious answer would be Theistic Evolution or Evolutionary Creationism. The reason you would adopt this view is because you believe that both the Christian worldview and Darwin's theory of universal common descent are true, so you would adopt a view which integrated two things that you know to be true.
It would be unreasonable for such a person to be a young earth creationist, since that would be ignoring the arguments for Darwinian Evolution that he finds so compelling. It would also be unreasonable for him to be an Old Earth (special) Creationist since this would also be ignoring the arguments for Darwinian Evolution to be so compelling. It would also be unreasonable for such a person to be an atheist, since this would be ignoring all of the non-biological arguments for theism that he finds so compelling. The most reasonable position for such a person to take would be Theistic Evolution. He's not adopting the model to keep theism from being falsified, but to integrate two different things that he holds to be true.
If you have evidence for God and Darwin, then mixing the two is perfectly rational. Now, if this person thought Darwinian Evolution were false, or if he thought even all of the non-biological arguments for theism were failures, then that would be different, but the person in this scenario doesn't believe these things. He believes the arguments for Christianity and for Darwinism are both successful. Such a person would be intellectually justified in adhering to Theistic Evolution on this basis.
Moreover, even special creationists would admit that sometimes God works through natural means. Hugh Ross ((and myself)), for example, argues that the collision event of Theia and Earth was how God decided to make the moon. And we believe this event was intelligently designed because of the various factors about this collision event that had to be just right in order for us to have moon capable of permitting life on this planet. Yet, although designed by God, this was a natural event. God didn't just *poof* the moon into existence. In the book of Exodus, God used a powerful wind to part the Red Sea (Exodus 14:21). It isn't unlike God to work through natural things. So a Theistic Evolutionist could argue that if God could use natural means for these aforementioned events, why couldn't He use nature to create living creatures? It's totally plausible to think that He could have used evolutionary processes if He wanted to.
Theistic Evolution is by no means an ad-hoc hypothesis. Most TEs you talk to will defend their view by appealing to evidence for both theism as well as for Darwinism. They believe TE because they're integrating two sets of truth.
*5 Reasons Why I'm Open To Theistic Evolution
*Why I'm Skeptical Of Darwinian Evolution
*How NOT To Argue Against Macro Evolution