Michael Torrice in Science:
Evolution doesn't make U-turns, according to a new study of proteins. The study shows that simply reversing selective pressure won't make a biomolecule revert to an earlier form. The finding confirms a much-debated biological law that, evolutionarily speaking, there's no going back.
Since the late 19th century, evolutionary biologists have debated whether evolution can go in reverse. If not, then evolution may depend on more than just natural selection. Multiple evolutionary paths could be possible through small chance events. It hasn't been easy to examine reversibility. Previous studies have focused on complex traits such as whale flippers, and scientists often lack sufficient information about ancestral traits or how present-day traits evolved.
So evolutionary biologist Joseph Thornton of the University of Oregon, Eugene, and his colleagues picked a more tractable subject: a single protein.