How can the number of genes increase through evolution?

Question

I am aware of the basics of evolutionary theory, however I don't understand how mutations can add genes over time. Am I correct in thinking that creatures within the same species who mutate to have an additional gene in their genome would normally be infertile? Or am I misunderstanding that?Can someone explain the process of the creation of genetic material through evolution? Citation to a decent academic paper or book on the matter would be appreciated too.

Answer 1

Two ways to get additional genes are gene duplication events and horizontal gene transfer. Adding new genes does not (necessarily) cause infertility. You are likely thinking of species having an odd number of chromosomes, which generally causes sterility.

In gene duplication, due to either an error in gene replication or other event (integration of an mRNA even) a gene is duplicated in the genome. This often frees the second copy from the selective pressure it was under, allowing it to evolve to perform other functions. It could also allows both copies to evolve to be more specialized, especially in the case of a parent gene that had multiple functions (as an example, the RNA polymerases in plants are thought to have arisen in this way http://symposium.cshlp.org/content/75/285.full.pdf).

In horizontal gene transfer, the new gene comes from another organism. One way for this to happen is through viruses (as an example https://www.cell.com/cell/fulltext/S0092-8674(17)31504-0). It can also happen between other interacting organisms.