Now, scouring the genome of a Japanese yeast, scientists have found a trackway of fossil genes in the making, providing a rare look at how an organism, in response to the demands of its environment, has changed its inner chemistry and lost the ability to metabolize a key sugar.
The finding is a snapshot of evolution at work showing, at the most fundamental level, how traits and features are discarded by virtually all forms of life when they are no longer needed.
"Many people think evolution is always happening in a forward direction," that new features are just tacked on, says Sean B. Carroll, a professor of molecular biology at the Howard Hughes Medical Institute at the University of Wisconsin-Madison. "The other side of the coin is that we lose things. Losses as well as gains make up the story of evolution."
Writing Sept. 20, in the Proceedings of the National Academy of Sciences, Carroll and colleagues Chris Todd Hittinger and Antonis Rokas describe the discovery of a set of seven genes caught in the act of fossilization.
In the Japanese yeast, known to scientists as Saccharomyces kudriavzevii, the Wisconsin scientists found the decaying genes that, when intact, are all functionally related and make up a pathway that permits the organism to convert the sugar galactose to energy.
The same pathway, among the most studied in biology, exists not only in yeasts, but also in virtually all other microbes, as well as plants and animals, including humans. Their sole function is devoted to the utilization of galactose, suggesting that the yeast experienced an ecological shift that somehow removed galactose from the organism's menu, thereby making the galactose-processing genes obsolete.