The "inverse relationship between evolutionary rate and age of mammalian genes" is an artifact of increased genetic distance with rate of evolution and time of divergence.

It has recently been claimed that older genes tend to evolve more slowly than newer ones (Alba and Castresana 2005). By simulation of genes of equal age, we show that the inverse correlation between age and rate is an artifact caused by our inability to detect homology when evolutionary distances are large. Since evolutionary distance increases with time of divergence and rate of evolution, homologs of fast-evolving genes are frequently undetected in distantly related taxa and are, hence, misclassified as "new." This misclassification causes the mean genetic distance of 'new' genes to be overestimated and the mean genetic distance of "old" genes to be underestimated.