Because different species have different variations, everyone's different. These differences can be useful in some cases and not useful in others. Like for instance, the gazel. Once upon a time the gazel was eating leaves. Then one day all the leaves dissapeared where he could reach it, Then his friend, a slightly bigger gazel with a longer neck came by and ate the leaves just a bit out of reach for the shorter gazel. Over time the short gazel's died and the larger gazel's could breed together producing larger gazel's every generation. The gazel's have been naturally selected (only the bigger gazel's exist) to survive due to their adaptations (their different heights).
