Protein-coding regions in a genome evolve by sequence divergence and gene gain and loss, altering the gene content of the organism. However, it is not well understood how this has given rise to the enormous diversity of metazoa present today.
Lopez-Bigas, N., De, S., & Teichmann, S. A. (2008). Functional protein divergence in the evolution of Homo sapiens. Genome Biology, 9(2). https://doi.org/10.1186/gb-2008-9-2-r33