Evolution does not mean God didn't have a purpose which came about. To ask how an eye could arise is answered, albeit quite well by science already, by saying, "Because God wanted it to." It's not either evolution or God, as comfortable as that familiar dichotomy is for many people.
God created a world to which He granted the freedom to make itself. This is necessary if it's actually going to be real and not just a simulation that God plays out as Grand Puppetmaster. He wanted real creatures who could freely choose to love Him.
"I would like to suggest, respectfully, that when God came to create the world, the Creator faced a dilemma. God is faithful, and the natural gift of the faithful God will be reliability in the operation of creation. However, reliability by itself could harden into mere rigidity, leading to a clockwork world in which nothing really new ever happened. God is also loving, and the natural gift of the loving God will be an independence granted to creation. Independence on its own, however, could degenerate into mere license, leading to a world of disorderly chaos.
I believe that the God who is both loving and faithful has given to creation the twin gifts of independence and reliability. These find their reflection in the fruitful interplay of chance and necessity in evolving cosmic history. Such an account gives a much more positive understanding of the role of chance. Monod and Dawkins like to apply to chance the adjective “blind,” suggestive of purposelessness and meaninglessness, but we do not need to be beguiled by their tendentious choice of words. The shuffling operations of happenstance are a way of exploring and bringing to light the deep anthropic fruitfulness with which the physical world has been endowed. Chance is “the search radar of God, sweeping through all possible targets of its probing.”
We must find a balance to the unacceptable views of God as divine puppeteer, pulling every string and making creatures dance to the divine tune alone and God as divine spectator who just set it all going and left the universe to get on with it. An evolutionary world is to be understood theologically as a world allowed by the Creator to make itself to a large degree. Yet this self-making takes place in a setting of finely tuned potentiality. Creation is not the starting off of something that is produced ready-made; rather, it is a continuous process.
Because continuous creation allows room for creaturly freedom within this process, the consequences will be lots of things that have come about “by chance” in the course of history. I do not believe that it was laid down from the foundation of the world that humankind should have five fingers - it just worked out that way - but I by no means believe it is pure accident that beings capable of self-conciousness and of worship have emerged in the course of cosmic history. In other words, there is a general overal purpose being fulfilled in what is going on, but the details of what actually occurs are left to the contingencies of history (this happening rather than that). The picture is of a world endowed with fruitfulness, guided by its Creator, but allowed an ability to realize this fruitfulness in its own particular ways. Chance is a sign of freedom, not blind purposelessness."
- John Polkinghorne