Evolution in nature by natural selection only modifies the structures present in the ancestors and favors the better adapted structures.
Explanation:
An engineer can design the bones and muscles of the forelimbs of given organisms as per their needs and give rise to new unique structures for each of these organisms.
On the other hand, evolution in nature allows the modification of existing traits by the accumulation of variations. The adaptive variations in the traits are then favored by natural selection. Therefore, forelimbs of mammals such as horses, whales, and bats have evolved by modification of the forelimbs present in their common ancestors.
The limitation of modification of existing structures does not allow the formation of a completely new structure.
Disruptive selection. Disruptive selection eliminates phenotypes near the average and favors the extreme phenotypes, resulting in increased genetic variation in a population.
