For a physiological system to improve, it must be exposed to a stimulus greater than it is normally accustomed to, such as a faster pace or a heavier weight; this is referred to as the principle of

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.