It's a question biologists have debated for years: Can evolution run in reverse, like a videotape played backward? Now a new study suggests that indeed, evolution can sometimes retrace its steps; organisms become more like their ancestors when faced with similar environmental conditions. But the work also suggests that some evolutionary changes are irreversible.
In human history, certain developments can be reversed. (In 1933, for instance, the U.S. repealed a 1919 constitutional amendment prohibiting liquor.) Biologists have debated whether evolutionary history can be flexible, too; some think traits can be regained when the environment changes, while others have argued that a species will never become what it was before. The current study shows that "always and never are both wrong," says evolutionary biologist Michael Rose of the University of California, Irvine.
The cast of this evolutionary movie consists of a fly strain called "Ives," which Rose's lab has grown for a quarter-century. Rose and Henrique Teotónio, now at the University Museum of Zoology in Cambridge, United Kingdom, set 20 populations apart and selected for one of four abilities: to reproduce late in life, to reproduce in mid-life, to reproduce early in life or to survive a long period without food. After about 50 generations in these environments, the groups of flies had each evolved in their own way, not just with regard to these traits, but also in physical aspects such as size.
Then came the rewind: The populations were returned to the original Ives environment. The researchers monitored their evolution by measuring a range of traits, such as development time, starvation resistance, fat content, body weight, and fecundity. Although a few characteristics didn't change back--even after 50 generations--"the overwhelming tendency is to revert," Rose says. In some cases, the previous adaptations took only 20 generations to undo. It's not clear why some features were easily reversible while others weren't, Rose says.
"It's a really exciting study, because it's a comprehensive look at a fundamental and old question," says Richard Lenski, an evolutionary biologist at Michigan State University in East Lansing. In the end, Lenski says, the question of whether evolution is reversible probably depends on the peculiarities of a population's history. As with humans, some constitutions may be more easily amended than others.