This has been reduced in size usually over time and has less important functions in some related organisms than in others

Vestigial organs

Explanation:

Vestigial organs are organs that that were once functional in the evolutionary past but ceased to function or have reduced in function over time.

Due to continued disuse, this organs ceased to function or adopted to less important functions.

A good example of a vestigial organ is the Appendix in the human body, as the argument goes; the appendix is a tail that reduced in size with time due to disuse.