Evolution may never reach an ultimate phase of perfection, but continue to make small improvements throughout time.
Richard Lenski of Michigan State University started growing cultures of the Escherichia coli bacteria back in 1988. Since then, over 58,000 generations of bacteria were born on the simple nutrient medium, a Michigan State University news release reported.
"When hiking, it's easy to start climbing toward what seems to be a peak, only to discover that the real peak is far off in the distance," Michael Wiser, lead author and MSU graduate student in Lenski's lab, said. "Now imagine you've been climbing for 25 years, and you're still nowhere near the peak."
The metaphorical "peak" the researcher was talking about refers to what is known as a "fitness peak." The peak occurs when a population finds such a great set of mutations that any new mutations would cause a decline in the strength and quality of the species as opposed to an improvement.
Linky's bacteria have not reached that point, even after living in a simple environment for about a quarter of a century.
Most species are exposed to changing environments which forces them to keep finding new mutations in order to adapt, but researchers had believed organisms would eventually reach a state of "perfection" if kept in the same environment.
Over the course of the 25-year experiment, the researchers froze samples of each generation of bacteria. These specimens could be brought back to life to compete against younger organisms to see how close they have come to the "fitness peak."
"There doesn't seem to be any end in sight," Lenski said. "We used to think the bacteria's fitness was leveling off, but now we see it's slowing down but not really leveling off."
Wiser believes the mathematical "power law" is relevant to this situation. The slope of the formula's function gets less steep over time, but never totally levels off.
"It was surprising to me that a simple theory can describe the entirety of a long evolutionary trajectory that includes initially fast and furious adaptation that later slowed to a crawl," Noah Ribeck, co-author and MSU postdoctoral researcher who built a power law model on the subject, said. "It's encouraging that despite all the complications inherent to biological systems, they are governed by general principles that can be described quantitatively."
The researchers are yet to see an end in sight.
"I call this the experiment that keeps on giving," Lenski said. "Even after 25 years, it's still generating new and exciting discoveries. From the models, we can predict how things will evolve - how fit the bacteria will become - if future generations of scientists continue the experiment long after I'm gone."
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