In the beginning the giraffe population was very plentiful and the desert environment was thriving.
But there was one problem with this thriving environment, the giraffes were dying quickly because their necks were not long enough to reach the trees, their food. There was only a few giraffes who could reach the trees.
So the giraffes that were tall enough to reach the trees were the ones that survive. They were the ones who had the next generations of giraffes and they passed their traits down to their offspring. That is when Jean Baptiste Lamarck developed his theory about the necks of giraffes.
So a man named Jean Baptiste Lamarck thought that if these giraffes with the long necks had children of the next generation they would have long necks which would then keep getting passed down. This was evolution.
So now all the future generations of giraffes with these adaptions keep have babies and they had these adaptations. Now this is descent of modification.
Now that the giraffe population was thriving the environment and food chain is back to normal. Because the giraffes are probably a substainable pat of the ecosystem if they would have died off that could have messed up an whole ecosystem.