Well, the first thing you need to understand to answer this question is Darwin's theory.
What happens is that, for example, a species of giraffe has a small neck, what happens to it is that this species can't get the food on top of the trees, and then, this species will most likely die in a couple of time. But through mutation, this species can generate another one that fits well into that environment, being capable to get food on the ground or even with a bigger neck, and by that, the survival of the species is guaranteed for now. Let's think about humans for a second, we are a species that went through evolution as well as all the species on earth right? But how did that happen? It's easy, if one of our ancestors for example, didn't have the right DNA/genes that gave us a feature that protected us from some diseases, we would probably be dead.
And be careful, a gene can be anywhere in a chromosome, that's why it's so hard to study and map it. The same thing happen to cells, if they dont adapt well, they die or adapt on the next generation.
