Answer: Survival of the fittest
Explanation: There where not many trees in the plains and with so many Giraffes roaming around with little predators, food was hard to come by. As the lower branches started thinning out, only the taller giraffes could reach the higher branches. When there was no food left, the shorter Giraffes started to die off, leaving only the taller giraffes. This is known as Survival of the Fittest because only Giraffes with the taller gene where able to survove. The shorter gene is extinct now becasue they no longer had food to live off of.