Dogs have a reduced non-functional digit on their paws known as a declaw

It is called a vestigial structure.

Vestigial structures are anatomical features that do not have a purpose in the present form of the species. It is homologous to other species' structures that is functioning purposefully.  

It is considered a proof of evolution.

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.