When organisms face a situation with limited resources and overpopulation, they begin to compete over the decreasing amounts of food, water, and space. The organisms will fight each other, and many will not live very long. The organisms with beneficial traits, such as longer claws, being taller, running faster, etc. will live longer than those without, and pass those traits on to their offspring, who will live longer, and so on. The changes will build up over time until the other organisms die out completely, or become a different species.
Take for example, a herd of horse-like animals, that eat leaves on trees in Africa. If the resources become limited, such as all of the leaves being eaten off the lower parts of the trees, it is likely that some organisms will begin to starve. The organisms with longer necks would be able to reach leaves still, and survive to produce offspring. Those organisms could eventually eat all the leaves on their part of the tree, and so only the longest of the longest necked animals would survive. Their necks could keep genetically stretching over a period of time, until all of the organisms had an extremely long neck, creating the giraffes.
Take for example, a herd of horse-like animals, that eat leaves on trees in Africa. If the resources become limited, such as all of the leaves being eaten off the lower parts of the trees, it is likely that some organisms will begin to starve. The organisms with longer necks would be able to reach leaves still, and survive to produce offspring. Those organisms could eventually eat all the leaves on their part of the tree, and so only the longest of the longest necked animals would survive. Their necks could keep genetically stretching over a period of time, until all of the organisms had an extremely long neck, creating the giraffes.
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