Long ago there was a population of short northern giraffes. They were located in the savannas, bushes, and forests of Kenya and South West Ethiopia. Lions, hyenas, and leopards preyed upon these giraffes. For food the northern giraffes ate different types of leaves and even sometimes fruit. Acacia trees that grew in their habitat were their main source of food, Giraffes in this population had short necks since the leaves on these trees grew so low.
The Acacia trees started to grow with the leaves higher up on the trees. This left the giraffes with shorter necks to not be able to reach their food source. Where as the giraffes born with long necks could reach the Acacia leaves.
Overtime this left shorter necked giraffes unable to reach the Acacia leaves. they slowly died off and became more rare in the northern giraffe population leaving most of those giraffes to have long necks.