For evolution to occur, there must be a preexisting DNA sequence. This may spur in your mind a chicken versus egg argument, but it must be noted that long stretches of new random sequence are not made by any natural mechanism. In saying this, you can make the conclusion that it is not possible for any gene to be entirely new. But, there are four prime ways in which innovation occurs, which you must be aware of. They are listed below, according to Alberts et al.
These four very important processes are commonplace and leave a notable trace in the DNA sequence of each organism in which they occur. In the diagram below, redrawn from the Alberts et al. text, are drawings of each of these processes in order to help you understand them better.
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