Giraffes not being able to reach their food.
Being "fit" is the only way to reproduce.
The ability to reproduce fertile offspring. Those who are "fit" will survive and pass down their traits, that is determined by the environment they live in
What will happen if giraffes end up with shorter necks?
In an ecosystem, some giraffes have long necks and others have short necks if something caused low-lying shrubs to die out, the giraffes with short necks would not get enough food. After a few generations, all of the giraffes would have long necks.
Giraffes could be selected out.
They could be selected out because most giraffes have long necks and the ones with short necks can get food from the the ones that have long necks, the giraffes that have long necks are going to over populate the ones with short necks