Why is the average collateral effect of synonymous mutations so similar across alternative reading frames?

The standard genetic code has been shown to have multiple interesting properties which impact on molecular biology and the evolutionary process. One facet of molecular biology where code structure is particularly important is the origin and evolution of overlapping genes. We have previously reported that the structure of the standard genetic code ensures that synonymous mutations in a protein coding gene will lead to a remarkably similar average "collateral" mutation effect size in at least four out of the five alternative reading frames. Here we show that only 0.26% of alternative codes with the block structure of the standard genetic code perform at least as well as the standard code in this property. Considering this finding within a code optimality framework suggests that this consistent effect size across the different frames may be adaptive. Here we give context for this finding and present a simple model where a trade-off between evolvability and robustness leads to an average mutation effect size which maximises population fitness. This supports the intuition that similar mutation effects across the different alternative reading frames may be an adaptive property of the standard genetic code which facilitates evolvability through the use of alternative reading frames.