Genomics Intelligent Design

What? Some viruses use an “alternative” genetic alphabet?

This is a problem, all right. But really, why do these, or any life/quasi-life forms, have a “genetic alphabet” (an alphabet of life, not learning) at all if everything happened by natural selection acting on random mutation, as the textbooks claim? Let alone an alphabet of life they can just substitute some other letters for? Is there anyone out there who can do the math?