We now know a lot about the dynamics of nucleotide substitutions within existing genes, but we've neglected one key component of molecular evolution. We haven't talked about where new genes come from. It's important to understand this phenomenon because, after all, new metabolic functions are likely to arise only when there are new genes that can perform them. It's not likely that an existing gene can adopt a new function while continuing to serve its old one.
Fundamentally the source of new genes is the duplication of existing genes and their divergence in function. As we'll see in a moment, for example, genes coding for myogblobin and hemoglobin in mammals are descendants of a single common ancestor. That's the duplication. Myoglobin is involved in oxygen metabolism in muscle, while hemoglobin is involved in oxygen transport in blood. That's the divergence. Although there are many interesting things to say about the processes by which duplication and divergence occur, we're going to focus on the pattern of evolution that arises as a result.