Evolution help with Biology!!?

as an example, lets propose that at one point there were such creatures as looked like giraffes but with short necks. Now, genetic mutations occur, causing some to have longer necks. This trait is ped from parents to offspring. This trait advantages the giraffe as it allows it to feed on the higher branches of trees, allowing it to obtain more food. As a result, the long-necked giraffes survive and reproduce, whereas the short-necked giraffes cannot obtain food and thus die off. therefore, over time, only long-necked giraffes are left.