The more mutationally robust proteins are more stable. The P450s from the polymorphic population neutrally evolved higher stability and expression levels than their counterparts from the monomorphic populations. The histograms show the distributions for the final protein from all monomorphic replicates and for the same number of randomly chosen proteins from the final polymorphic population. The plots show (left to right) the temperature at which half the protein irreversibly denatured after 10 min (T
50), the urea concentration at which half the protein denatured after 4 h ([urea]50), and the expression level relative to that of the original parental P450. The means are significantly different, with unequal variance t-test P-values of 0.02, 0.005, and 0.04, respectively.