In different lineages the toolkit of Hox genes has evolved independently. For example in vertebrates the whole group has been duplicated twice and all we vertebrates have four copies of the Hox gene cluster, although some copies have been lost as they did not evolve any new functions. When this happens natural selection does not act to maintain the gene because it does not affect the animal's fitness. The gene is left to accumulate mistakes until it is no longer used. This has been seen in nematodes (a group of worm-like animals); their Hox gene toolkit has been reduced, presumably because they evolved simpler body plans so that some of their Hox genes were no longer needed.