How does evolutionary theory explain the formation of vestigial structures?

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Natural selection acts to reduce structures over time, so eventually most structures get smaller and may no longer function.

Vestigial structures form if traits in two unrelated groups of organisms that live in similar environments come to resemble each other over time.

Even if a structure has a function, an accumulation of random mutations can cause it to become a vestigial trait.

If natural selection no longer acts on a structure because it serves no function, that structure may degenerate over time.