Four million years ago, horses were rather small compared to today's horses and had relatively stocky bodies with a straight shoulder and thick neck. This statement is an illustration of which biological concept

The biological concept of evolution.

Explanation:

The process by which living organisms defy the environment that they habit and shapes every single trait of them. According to evolution every species that has ever existed in this world has suffered modifications by different reasons (mutation, for instance) and environment was the responsible to select the ones that were more suitable to last. The process of evolution can be delusional because it may be necessary millions of years to fulfill its mechanisms, specially in complex organisms like mammals.