Evolution is one of those enigmatic subjects we simply do not understand fully. We know it is a biological change at the genetic level that changes the overall nature of an organism. We're also sure that it requires a significant number of generations. Unfortunately, that means it is difficult to observe and can easily be challenged by those with alternate viewpoints.
In the human context, evolution has been impossible to identify. We've only been able to decipher the genetic code for a decade yet it is estimated that millions of years might be needed to observe any significant event. This quandary has left many an evolutionary biologist frustrated.
There is another option to understand and elucidate evolution. By turning to our microscopic friends, such as the bacterium Escherichia coli, there is much that can be learned. Unlike humans, who need on average 20 years between generations, these small creatures only need about 20 minutes to foster offspring. This means that the bacterium can evolve some 500,000 times faster than us. For those studying evolution, this rapid rate offers the prime opportunity to explore some of the deeper mysteries and perhaps even offer mechanisms to best predict when evolution might happen to us.
The examination of E. coli evolution was heralded back in 1994 when researchers from Michigan State University followed the changes in the bacterium over 10,000 generations (the equivalent of 200,000 human years) . What they found was that the majority of change happened in the first few thousand generations as the ability of the organism - its fitness - grew. But once an optimal level of fitness was achieved, the bacteria stopped evolving. Although no genetic analyses were performed, there was reason to believe that evolution occurs in fits and starts.
It took another 15 years before the same researchers were able to provide some perspective at the genetic level. By this point, some 40,000 generations had been logged, equating to some 800,000 human years. For the majority of bacteria, a few insignificant changes were observed - akin to a change in height in humans - but overall, there had been few major changes in the way the bacteria lived.
As they scoured through the population, they found only one bacterium, out of the trillions who had lived and died, that had undergone a genetic change. It occurred around 20,000 generations -- 400,000 human equivalent years. What was truly surprising about this newly evolved bacterium was the fact its descendants went through a high frequency of evolutionary events over the following few thousand generations. Then, after about 2,000 or so generations, the changes settled down. This was no different than what had been observed when the bacterial culture had been started.
The observation helped to prove that the majority of changes occurred in the early part of evolution and slowed over time as the organism becomes stable. The data also helped to reveal that once a new shift occurs, the process was the same, regardless of the 'age' of the organism. Yet even this could not resolve one piece of the puzzle. Is evolution simply a spontaneous event or could it be forced?
Classically, evolution is thought to occur in response to a change in the environment. Whenever there is a long-lasting change in the food source, the climate, the presence of new chemicals, and population density, there needs to be a means to cope. The only option is to change biologically, through the process of genetic mutation. When the DNA changes, so does the body. However, there was no indication if the change was spontaneous or actually due to pressure. Even more important was whether evolution was always positive or if there were negative consequences.
This week, a possible answer was revealed. Using the same E. coli as in the 1994 and 2009 studies, a team of researchers from Harvard University examined the impact of environmental changes on bacteria that had been grown for both 20,000 and 50,000 generations. The group figured that after what would be close to a million human equivalent years growing with the same food source and climate, changes would lead to several evolutionary events. What they found, however, was a picture of evolution that they simply did not expect.
When the food source was modified, both the 20,000 and the 50,000 generation bacteria underwent several changes. However, the younger population did significantly better than the older as their mutations were beneficial; the older population had mainly detrimental adaptations. The same difference was seen when the climate changed. The younger bacteria were able to deal with a shift in temperature and survive quite well. The older, 50,000 generation microbes, sadly, were not as successful. It was clear that environment did have an effect on evolution yet could only be forced in younger populations; as the species ages, it becomes less able to deal with change.
The overall results of the three studies suggest that evolution is a rather complex process that cannot be simply explained by a two-and-a-half hour debate. We are just beginning to understand the mechanics of evolution and its impact in the short and long term. However, by adhering to what has been seen in E. coli, we can surmise that the time for grand human evolution is long behind us and we are only now settling in to a comfort level. While there may still be room for more adaptation in the future, the likelihood is that it won't do us much good and may even harm us. Not to mention, should there be any significant environmental change, we may find ourselves left with few evolutionary options, at least biologically.
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