The species of the "Moldy" rat is new and upcoming. They live in most dry areas and usually in holes in the ground. Their biggest predator is the "Fright" vulture, they swoop down and hunt the rat by grabbing it by its tail.
As a new species, the rats have found food to eat, extra grains dropped by wheat in the fields. With this supply of food, the rats have come a long way in reproduction and have reproduced many offspring. Overpopulation has begun.
With so many rats being born, not all of them are receiving the same traits as the producers, causing some variation throughout the population. Not only are there dark colored rats, but light colored, long tailed, and short tailed.
The vulture only has the skill to pick up the rats with the longer tails, so the rats with shorter tails are living much longer than other. Because of this, majority of the rats that are living have short tails. This is the process of selection.
The rat species, genetically, had a long tail following it when it travels. With the vulture hunting it down, it is easily caught because its tail is so long. The rats caught onto this pattern of hunting by the vulture, and slowly, as each rat started surviving some frightening catches, their tails became shorter each catch they survived.
So, with the rats losing parts of their tails, they have started reproducing baby rats with shorter tails. This is an adaptation occurring because of the food chain. When this starts to happen the rats are living longer, and are decreasing their chances to be hunted down by the vulture.