Under the assumption that silent substitutions   are largely immune from selection and accumulate at a stochastic rate that is   proportional to time, we take the number of substitutions per silent site, S,   separating two members of a pair of duplicates to be a measure of the   relative age of the pair. Letting R denote the number of substitutions per   replacement site, a net (cumulative) selective constraint since the time of   origin of a pair of duplicates will be reflected in an R/S ratio < 1,   whereas a net acceleration of protein evolution will be revealed by an R/S   ratio > 1. Complete relaxation of selection will result in R/S ≈ 1. For the duplicate genes   that we have identified, there is often considerable scatter around the   neutral expectation when S < 0.05 (Fig. 1), suggesting that early in their   history, many gene duplicates experience a phase of relaxed selection or even   accelerated evolution at replacement sites. The progressive decline of R/S   beyond this point reflects a gradual increase in the magnitude of selective   constraint. The vast majority of gene duplicates with S > 0.1 exhibits an   R/S ratio << 1.

假设沉默突变在很大程度上不受选择的影响，并且以与时间成正比的随机速率累积，我们取每个沉默位点的替换数为S。将S作为一对重复基因分化相对时间的度量。如果R表示每个错义突变位点的替换数量，则自一对重复基因产生开始的净（累积）选择性约束将反映为R/S <1中，而蛋白质进化的发生由R/S >1表示。完全的松弛选择将导致R/S≈1。对于我们已经鉴定的重复基因，当S<0.05（图1）时，通常在中性预期周围有相当大的散点分布，这表明在它们的历史早期，许多基因重复经历了一个宽松的选择阶段，甚至在错义突变位点加速进化。超过这一点的R/S的逐渐下降反映了选择性约束的程度逐渐增加。当S>0.1时绝大多数基因重复序列的R/S比小于1。