Darwinian Selection In The Classifieds

Using comparative sequence analyses, we can identify proteins that may have been subject to positive darwinian selection. To test these statistical results, it is important to develop functional assays and identify amino-acid changes that are responsible for the adaptation of organisms to specific environments. One of the two duplicated pancreatic ribonuclease genes of a leaf-eating colobine monkey, douc langur, is now shown to have adapted to digest bacterial RNAs in the monkey's foregut.

Organisms encounter a diverse array of habitats, from the Himalayas to the deep sea, and adapt to these environments with an equally diverse array of structures and functions. One of the major goals of evolutionary biology is to elucidate mechanisms that drive these adaptive changes at the DNA, protein and functional levels.

In vertebrates, adaptive evolution has been extremely difficult to study, owing to the scarcity of genetic systems in which the functional effects of mutations can be evaluated experimentally. In principle, we can pick proteins with polymorphic amino acids and evaluate selective differences among them. In practice, however, the magnitudes of selective differences are generally so small that it is almost impossible to detect any significant differences in fitness1. Alternatively, adaptive evolution is strongly suggested by one type of protein having a strong association with a specific environment. Once a protein is suspected to be under positive darwinian selection, we can identify potentially important amino-acid replacements that may be responsible for adaptation. These hypotheses can then be tested experimentally (see figure). On page 411 of this issue, Jianzhi Zhang and colleagues2 report such a functional assay and present strong evidence of the adaptive evolution of a duplicate pancreatic ribonuclease gene in the leaf-eating colobine monkey, the douc langur.