The genetic code is redundant, there are 20 amino acids for 64 possible nucleotide combinations (triplet codons). Therefore some amino acid are coded by several different codons. While leucine is coded by 6 codons, tryptophan is coded only by one codon.
[I am aware that the set of codons that code for one given amino acid tend to look alike each other more than random. Usually it is only the last base that does not affect the amino-acid that is encoded.]
I therefore do not think that the genetic code can be entirely be explained by “it happened to occur that way the first time” (at the origin of life or in the last universal common ancestor) “and it never changed”.
So, my questions are:
Why are some amino acids coded by a several codons while others are coded only by one or two?
Specifically, why is methionine coded by only one codon — AUG — which has also to serve as a start signal?
In general, how (by what mechanisms, selective pressures) has the genetic code evolved to give this pattern of redundancies?