Sequence and Functional Conservation
The homeodomain protein motif is highly conserved across vast evolutionary distances. In addition, homeodomains of individual Hox proteins usually exhibit greater similarity to homeodomains in other species than to proteins encoded by adjacent genes within their own Hox cluster. These two observations led to the suggestions that Hox gene clusters evolved from a single Hox gene via tandem duplication and subsequent divergence and that a prototypic Hox gene cluster containing at least seven different Hox genes was present in the common ancestor of all bilaterian animals.
The functional conservation of Hox proteins can be demonstrated by the fact that a fly can function perfectly well with a chicken Hox protein in place of its own. This means that, despite having a last common ancestor that lived over 670 million years ago, a given Hox protein in chickens and the homologous gene in flies are so similar that they can actually take each other's places.
Read more about this topic: Hox Gene
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