People seem to think that our DNA has some sort of conciousness, and when the conditions change, so do the genes. But that is not what evolution says. The mutations are completely arbitrary. For example, Giraffes. If for some reason, one giraffe is born with a mutation that gives it a longer neck than the others; then there is a draught which creates a food shortage. Well, the giraffes with longer necks are going to have a better chance of survival, as they will be able to reach higher branches on trees to obtain food. Genes do not mutate because of a change in conditions, but a change in conditions can be the reason some genes are passed on more frequently than others.