THE SHEEP ARE DIFFERENT ENOUGH TO PICK FROM.
Variation is the differences in animals. It is your potential to change. For example, when you look down a sheep race, the animals are different sizes representing the variation in body weight. You have the ability to select for heavier animals because of that variation.
The potential to improve or manage a trait depends on how much of that variation is due to genetics.
If you select from the top 10% of animals then the progress (grey arrow) depends on the genetic variation available.
More variation means more potential to change