Hox genes are a group of regulatory genes that control the timing and route of development. They’re a bit like lighting engineers at a concert; they control when and where a light goes on, how strongly and for how long it is switched on. They all cluster together on the genome and play a major role in the development of animal segments with different Hox genes being expressed in different segments. Hox gene evolution played a big role in the diversification of these segments. But how do we know this?
Homologous structures are things that are similar because they evolved from one common ancestor. For example the bones in your arm are homologous to the bones in a mouse’s foreleg. In the same way genes in different animals can be homologous. This is the case for Hox genes. Biologists have been able to isolate Hox genes from many animal groups.
By looking at which Hox genes are present in many different animals it is possible to track their evolution. The number ofHox genes has actually expanded during animal evolution. Jellyfish and other cnidaria have only two Hox genes. Sometime after they split from the rest of the animals these genes duplicated – this means when the DNA was being copied in the sex cells of our ancestors mistakes were made, and instead of copying the Hox genes once they were copied twice. In fact the bilateria have at least seven Hox genes, meaning they were duplicated more than once.
When a Hox gene was duplicated, the functions of the two copies diverged and each evolved a special role in the development of the animal. However some sections of the gene remained the same – they were conserved – these allow us to see how the genes are related.
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 Hoxgene toolkit has been reduced, presumably because they evolved simpler body plans so that some of their Hox genes were no longer needed.