Early convincing evidence of this definition of neutral mutation was shown through the lower mutational rates in functionally important parts of genes such as cytochrome c versus less important parts and the functionally interchangeable nature of mammalian cytochrome c in in vitro studies. Nonfunctional pseudogenes provide more evidence for the role of neutral mutations in evolution. The rates of mutation in mammalian globin pseudogenes has been shown to be much higher than rates in functional genes. According to neo-Darwinian evolution, such mutations should rarely exist as these sequences are functionless and positive selection would not be able to operate.