Chapter 7 of my book Information theory and molecular Biology discusses
the evolution of the standard triplet genetic code from a primitive
doublet code. The mathematical background needed to understand the
argument is in Part I of this book.
Chapter 7 shows that if the primeval genetic code had a vocabulary of
fewer than 20 amino acids, for example perhaps 8, then there should have
evolved about 10 separate codes. These are the mitochondrial genetic codes
usually regarded as minor differences from the standard genetic code.
When you read Chapter 7 please e-mail me any comment or questions.