I've been wondering lately how evolution manages to produce complex organs. It is pretty obvious to me how evolution would select some minor traits like size, resistnce to illness or climate. There is a mutation where some organisms are slightly different and better traits are selected. However I cannot imagine how an organism will mutate to have lets say lungs at once and unless organism has a functional organ there is no reason for the trait to be selected. So how does evolution produce complx organs?
The best introduction to evolution of complex phenotypes via master control genes is Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom by Sean B. Carroll. You might also want to have a look at a more recent Evolutional developmental biology (Evo Devo) book like those you get when searching for "Evo Devo" in Google Books.
In summary, it is possible for a mutation to change a shape by changing the spatial distribution of a transcription factor during embryo development. What's more, by changing the distribution of two mutually dependent factors, formation of any shape is possible if the distributions overlap. Such transcription factors are highly conserved in evolution and the most important cause of the endless forms we see.
It can't be explained by evolution, at least not very well. There is zero evolutionary advantage for an organism to have half of the characteristics needed for vision, in fact, multiple half formed mechanisms would be a huge detriment. That said, consider the hundreds of the complex systems present within humans? Adding time to the equation does not solve this fundamental paradox. If one of those mechanisms in a complex system does not work, the system does not work, if one of the systems in a human body does not work, it is like the person does not live (consider that of the hundreds of systems within a human body, most are essential to sustaining life). As a medical doctor I can attest to how unbelievably complex the human body is. I am not saying there is a god, but I also can't believe that time and mutation were enough to provide for the complexity we see in humans and other organisms.