No, your question was just oddly worded. Now it is extremely broad. Shall I narrow it down a bit, to the evolution of the eye?
While some people will try to claim that a structure such as the eye is irreducibly complex; that is, if any genes contributing to it were not present, the resulting structure would be useless, and thus there would be no selective pressure promoting the intermediates between having eyes and not having eyes.
This is actually incorrect, for a few reasons, though it is an understandable mistake. I will go through with you as to how eyes evolved, and mention the errors made in that assumption along the way.
Eyes started out as merely light sensitive portions of eukaryotic cells. Even modern single-celled eukaryotic organisms often have these, and the "eye-spot", as they are often referred to, is actually the most common eye in nature. Here's a wiki article on how they work, feel free to use the sources they cite for further reading
https://en.wikipedia.org/wiki/Eyespot_apparatus .
A single gene mutation easily accounts for these simple eyes, which only detect light intensity.
These eye spots are a cell organelle, and thus the cell body can make multitudes of them to promote a stronger response from light. From the start, they cluster in one area of the cell, much like how our ocular organs are in one location and not all over the place. This forms a patch of light sensitive cells.
One of the nice things about eye evolution is that creatures with intermediate eyes still exist. There currently are organisms that have just a patch of light sensitive eyes. From this point forward, I will go easy on myself, because this topic has enough information to fill multiple books and I am not typing that all out. So, I will link images of creatures that have the intermediate eyes, and a short statement indicating what the intermediate represents.
http://www.pnas.org/content/104/suppl_1/8567/F1.large.jpg all next to each other as a diagram of eye structure.
http://www.d.umn.edu/~olse0176/Evolution/bac_bacteria2.jpg eyespot
https://kelltrill.files.wordpress.com/2010/09/planarian2.jpg pigment cup eye (isn't it interesting how even at just having a couple of layers of photoreceptive cells, that these simple structures share asthetic similarities with our eyes?)
http://www.d.umn.edu/~olse0176/Evolution/pin_naut.jpg pinhole lens eye (start of eye structures only seen in multicellular organisms, however, it should be noted that there are some multicellular organisms with pigment cup eyes, such as mollusks)
http://www.detectingdesign.com/images/HumanEye/humane6.jpg http://www.snailfarmingsecrets.com/wp-content/uploads/2015/04/Helix_aspersa.jpg close-up and normal view of a garden snail eye, which is an eye with a primitive lens.
http://warrenmars.com/photography/technical/resolution/human_eye_diagram.gif a complex eye. Note, however, that humans are far from having the best vision of any creature.
You might wonder why all these stages of eye development continue to exist. There are a few reasons for that. For one thing, light detection is such a beneficial trait to have, that it has evolved independently a minimum of 50 times at various points in history. Hence, some lines of eyes are evolutionarily behind others. Furthermore, eyes are a costly structure to maintain, so single-celled organisms have limits as to how developed their eyes can be before they become more of a hinderance than a benefit. This can go to multicellular creatures as well, hence why nocturnal creatures either have extremely large and developed eyes, or very small, underdeveloped eyes. Predators benefit more from being able to see distinct colors than prey animals, and prey animals benefit more from detecting motion and having a wide field of vision, hence their differences in eye structure and position on the body. Humans have predator-style eyes.
