Contraction is the answer
To retain a better sense of balance or aid in reaching limbs if a tree living animal. Also, too protect from predator attacks from behind.
Mutation can be both beneficial and harmful.
A very common beneficial mutation is camouflage and natural selection. The organism will acquire the change and pass it on to their offspring.
A harmful mutation can be diseases or a common harmful mutation, Down syndrome. Which gives a person an extra chromosome.
