Errors in DNA replication can lead to duplications of sections of DNA or, as in the creation of polyploids, duplication of the entire genome (see Chapter 4). It's akin to going to Kinko's for a single copy and ending up with two: the copy you needed and an extra you're not quite sure what to do with. The same thing goes for the DNA. What's an extra gene good for?
If a mutation knocks out a duplicate copy, the organism is pretty much back where it started: It still has the original copy and an extra. This situation is the spare-tire gene duplication theory. (Just kidding.) The extra copy may or may not be functional:
^ When the second copy is nonfunctional: These nonfunctional genes are called pseudo-genes. Pseudo-genes look like the original genes, but they're a little, or a lot, broken, and they tend to accumulate more and more mutations over time. The more mutations occur, the harder it is to recognize that these genes are related to existing genes. But since they're already nonfunctional, there's no fitness cost to a few more mutations.
^ When the second copy is functional: If the second copy mutates such that it is able to perform an additional function that's selectively advantageous, individuals with this mutation increase in frequency, resulting in two different yet related functional genes where one originally was. Many examples exist of families of genes that appear to have a common ancestor.
^jjjWJEft When you know about the possibility of gene duplications, you understand that natural selection could result in a change of function of one copy without having to worry about losing the function of the other copy. When you have two copies, you have room to move. Because most mutations are deleterious, these duplicated genes usually end up losing their original function without acquiring any new function. But sometimes beneficial mutations occur in the second copy, and a new gene is born.
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