Final answer:
Short-necked giraffes may have evolved into giraffes with longer necks through natural selection. Giraffes with longer necks were better able to reach leaves and had a greater chance of survival and reproduction, passing on the long-neck trait to their offspring. Over time, all giraffes ended up having long necks.
Step-by-step explanation:
In the case of giraffes, the ancestors with shorter necks may have evolved into giraffes with longer necks through the process of natural selection, based on the principle of survival of the fittest. Initially, variations in neck length existed within the population, with some individuals having slightly longer necks. When the environment changed and leaves became scarcer, giraffes with longer necks had an advantage as they could reach leaves other giraffes couldn't. Consequently, they were more likely to survive and reproduce, passing on the long-neck trait to their offspring. Over time, generations of giraffes with longer necks became more dominant, resulting in all giraffes having long necks.