There was once a time where there were short necked and long necked giraffes. The taller giraffes were always able to eat the tree leaves with no problem. However the short necked giraffes always struggled to reach the leaves so they starved
All my fellow short necked giraffes have died of starvation but the tall ones have survived.
Eventually, all the small necked giraffes died out and the long necked ones reproduced and had more long necked giraffes.
The giraffes reproduced a lot and now these are the giraffes we have now
Since there are short necked giraffes and there are long necked giraffes, this shows variation. The long necked giraffes are most likely to survive since they can reach high up on the trees. They are all competing for survival. The genes of having tall necks are passed down to their kids and the tall necked giraffes will dominate the short necked ones. The tall necked giraffes were naturally selected and they live on cause so they can survive in their specific environment.