The combination of natural selection and random mutation causes our DNA to change. You are probably focusing too much on individual animals rather than on the populations of animals. In any population you will have a range of traits. Let's take the giraffe example. Any population of giraffes will have a range of neck lengths among other traits. If a change in climate occurs and lower level brush gets scarce the taller, higher reaching giraffes can do better and will have more successful young. The shorter giraffes do not reproduce as well. Sooner or later the average height is higher. During all of this time random mutation has been going on so in each generation there are going to be changes that increase the heights of those individuals. Some of these height changes will be better than others, there are always side effects to change, and again the better changes will out produce when it comes to raising young.<quoted text>
LMAO! where did you get that theory about giraffes? Darwin from the 1800? LOL.
How does our DNA change? You have an explanation? or sleepiness is your excuse?
Random variation and natural selection is a very powerful and simple tool for change.