May 19, 2006 2:08 PM PDT
Is evolution predictable?
Associate professor Yousif Shamoo and two students recently conducted experiments on a microbe, G. stearothermophilus, to see how it adapted to different environmental circumstances. In the experiment, the dominant strains of separate generations of the microbe ended up developing the same mutant gene in response to the same environmental hazards.
Conceivably, if scientists can predict how the microbes will adapt to changes in their environment, they can develop antibiotics that won't be rapidly rendered ineffective by stronger, successive generations. In other words, if researchers can figure out what gene might evolve in response to a medicine, they can figure out a way around that response.
In the experiment, the team created a mutant strain of the microbe that was unable to live in high-temperature environments. Typically, the bacteria can continue to thrive when the temperature hits 73 degrees Celsius (163 degrees Fahrenheit). The experimental strain of bacteria contained a mutated version of a gene that, in the naturally occurring strain of the microbe, produces a protein that made existence possible.
They then put these mutant strains in environments where the temperature rose slowly but steadily, and studied how different generations coped with the changing temperature.
In the breeding that followed, millions of new mutations of the gene in question were produced, but only about 700 of those variants replicated some of the functionality of the naturally occurring gene.
One variant, called Q199R, appeared almost immediately, and the bacteria that contained it became the dominant strain of bacteria through 500 generations of breeding. The gene, however, couldn't provide protection after 62 degrees Celsius.
At that point, five new strains of bacteria, all with slightly different versions of Q199R, appeared. Three of the five new strains were driven to extinction in a few days, while the remaining two fought it out for three weeks longer.
The group then conducted the experiment again, and the same mutations developed. Thus, the experiment suggests that evolutionary development can be predicted, the researchers said.
"The duplicate study suggests that the pathways of molecular adaptation are reproducible and not highly variable under identical conditions," Shamoo said in a statement. "One of our most surprising findings is that an estimated 20 million point mutations gave rise to just six populations that were capable of vying for dominance. This suggests that very few molecular pathways are available for a specific molecular response."
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