This paper provides evidence for independent parallel evolution at the protein sequence level of RNases in Afican and Asian Colobine monkeys who need extra Rnases to digest bacterial RNA. The author establishes four criteria for demonstrating parallel adaptive evolution:
I propose that four requirements should be fulfilled to demonstrate adaptive parallel evolution at the protein sequence level. First, similar changes in protein function occur in independent evolutionary lineages. Second, parallel amino acid substitutions are observed in these proteins. Third, the parallel substitutions are not attributable to chance alone and therefore must have been driven by a common selective pressure. Fourth, the parallel substitutions are responsible for the parallel functional changes.
I can imagine many other research possibilities, not only for this gene in New World leaf eating monkeys, but also in other systems in humans and other organisms. In humans: parallel evolution for lactose tolerance, high altitude...
Abstract: Similar morphological or physiological changes occurring in multiple evolutionary lineages are not uncommon. Such parallel changes are believed to be adaptive, because a complex character is unlikely to originate more than once by chance. However, the occurrence of adaptive parallel amino acid substitutions is debated1, 2, 3. Here I propose four requirements for establishing adaptive parallel evolution at the protein sequence level and use these criteria to demonstrate such a case. I report that the gene encoding pancreatic ribonuclease was duplicated independently in Asian and African leaf-eating monkeys. Statistical analyses of DNA sequences, functional assays of reconstructed ancestral proteins and site-directed mutagenesis show that the new genes acquired enhanced digestive efficiencies through parallel amino acid replacements driven by darwinian selection. They also lost a non-digestive function independently, under a relaxed selective constraint. These results demonstrate that despite the overall stochasticity, even molecular evolution has a certain degree of repeatability and predictability under the pressures of natural selection.