In “Evolution of increased complexity in a molecular machine”(Nature 481, 360–364 (19 January 2012) doi:10.1038/nature10724), Gregory C. Finnigan, Victor Hanson-Smith, Tom H. Stevens & Joseph W. Thornton offer, according to W. Ford Doolittle, “the most compelling experimental evidence to date”:
Our experiments show that increased complexity in an essential molecular machine evolved because of simple, high-probability evolutionary processes, without the apparent evolution of novel functions. They point to a plausible mechanism for the evolution of complexity in other multi-paralogue protein complexes.
From W. Ford Doolittle (1/19/12 issue):
… A general theory, sometimes called constructive neutral evolution, to explain how neutral processes might drive a system towards complexity is more than a decade old. But the study by Thornton and colleagues may provide the most compelling experimental evidence to date. Of course, one can never prove that some subtle, unidentified selective advantage was not involved in the evolution of the V-ATPase protein ring, but neutrality would seem the most justifiable default hypothesis. Thus, a neutral theory of molecular evolution, normally invoked for nucleotide substitutions, may also apply to certain higher-order structures such as multi-subunit protein rings. How general such neutral mutational drives to complexity might be is one of evolutionary theory’s deeper unanswered questions.