Past collaboration by Adami and experts in bacteria has indicated that the computer simulations are realistic. So Adami thinks a living colony of asexual bacteria would probably respond much like the computer-generated organisms when subjected to comparable kinds of actual stress.
Lingering Mystery
The study builds on earlier work by Adami and others showing that digital organisms can adapt to become more robust, or less vulnerable to harm by environmental changes and other factors.
"What we showed in the other paper," said Adami, "is that if you transfer a fragile organism that evolved with a small mutation rate into a high-mutation-rate environment, it will adapt to this environment by becoming more robust."
The origin of sexual reproduction has been a mystery in part because of an effect known as "mutation accumulation." This means organisms tend to adapt in ways that reduce the effects of mutations, thus making the organisms less vulnerable.
But this kind of robustness is actually deleterious, the researchers contend, because harmful mutations would accumulate in the organism through sexual recombination, leading to a gradual loss of genes. This handicap of sexual creatures would be enough to guarantee their extinction when competing against asexual ones.
This could be avoided if mutations were unable to accumulate. Wilke and Adami propose that through a "conservation law," an organism may be able to withstand the harmful effects of a few mutations but incapable of surviving a large number of mutations.
As organisms become increasingly resistant to the effects of single mutations, they are unable to tolerate multiple mutations. This removes organisms with multiple harmful mutations from the population; sexual recombination allows the organisms to reap the rewards from sharing beneficial mutations.
Stressful environments that cause high mutation rates make organisms more robust—or less vulnerable—to the harmful effects of single mutations. The researchers say their proposed conservation law ensures that this evolutionary pressure puts asexual organisms on to the road toward sexual recombination.
The published article is available online at: www.pubs.royalsoc.ac.uk/proc_bio/proc_bio.html
