  Genetic robustness, the preservation of evolved phenotypes against genotypic
mutations, is one of the central concepts in evolution. In recent years a large
body of work has focused on the origins, mechanisms, and consequences of
robustness in a wide range of biological systems. In particular, research on
ncRNAs studied the ability of sequences to maintain folded structures against
single-point mutations. In these studies, the structure is merely a reference.
However, recent work revealed evidence that structure itself contributes to the
genetic robustness of ncRNAs. We follow this line of thought and consider
sequence-structure pairs as the unit of evolution and introduce the spectrum of
inverse folding rates (IFR-spectrum) as a measurement of genetic robustness.
Our analysis of the miRNA let-7 family captures key features of
structure-modulated evolution and facilitates the study of robustness against
multiple-point mutations.
