Darwinian evolution entails more than just the concept of one species changing into another over a long period of time.  It involves a fully naturalistic process: natural selection working on random genetic mutations, genetic drift, etc.  If Darwin’s theory of evolution is scientifically sound—meaning the naturalistic processes it invokes are fully capable of producing life and all of its many variegates—then adding God to the equation is superfluous.  It would be like providing a scientific account of water boiling by saying water will boil at time t1 when X amount of heat is applied to Y amount of water at Z altitude, but then adding that fairies are also involved in the process.  If naturalistic processes are adequate to explain why water boils, then not only is there no need for the fairy hypothesis, but there is no room for it.  The same is true of Darwin’s theory of evolution.  If the theory is scientifically sound, and naturalistic processes can fully account for all of life, then there is no need for, and no room to fit God into the picture.  In other words, if Darwin’s theory is scientifically sound, positing a theistic form of evolution is superfluous.

One might say, however, that naturalistic processes are not fully adequate to account for all of life, and this is why one must add God to the equation to make it work.  To make such a claim, however, is to admit that the scientific theory itself is not sound on its own.  It requires some outside supernatural force to patch it up.  Here’s the rub: If Darwin’s theory of evolution is not adequate in itself to explain the data, why should we feel compelled to fit theism into the picture?  Let’s face it, the only reason a theist would postulate a theistic form of evolution is if he was convinced that the evidence for evolution was so compelling that intellectual honesty demands that he reconcile the scientific evidence with his theistic belief.  But if Darwin’s theory of evolution lacks the evidence necessary to make it a sound scientific fact, what compelling reason is there to reconcile the theory with theism?  If Darwin’s theory is not sound in itself, it doesn’t need God to shore it up.

For further reading see my article titled “Theistic Evolution: The Illegitimate Marriage of Theism and Evolution”