Lenski and 50,000 generations of E. coli

Richard Dawkins didn’t mention it during his visit to New Zealand, but a long-running experiment that most clearly demonstrates how evolution works celebrated a major milestone last month.

Since 1988, at his lab at Michigan State University in East Lansing, Michigan, evolutionary biologist Richard Lenski has been running an ongoing experiment that demonstrates on a small scale how genetic mutations and natural selection work over successive generations – giving clues as to how us humans have evolved.

The so-called E. coli long-term evolution experiment focuses on 12 beakers containing bacteria grown from one original E. coli bacterium and that have been carefully nurtured for around 22 years. The idea is that because bacteria reproduce so quickly, changes over thousands of generations of bacteria could be observed in a relatively short period of time. Dawkins says the reason people struggle to get their heads around evolution is because, for species of large animals like mammals, evolution happens incredibly slowly – noticeable changes take millions of years to manifest themselves. Here then was an opportunity to watch evolution in fast-forward.

e coli
Lenski's experiment - look no further for evolution in action. Source: Wikipedia

The 12 beakers of bacteria are fed daily with glucose to nurture the populations of bacteria. This is a ritual that has been carried out since day one and last month, on February 14th, the 50,000 generation of bacteria developed in the beakers. Through the years, Lenski has been able to observe how these separated populations that came from the same source have grown – and in the process made some startling discoveries.

Many of the populations developed in the same way as you would expect with the uniform environment and diet. All of the populations grew faster with successive generations until around the 20,000th generation when growth levelled off at 70 per cent faster than the original strain. But along the way, as is the nature of evolution, the bacteria underwent numerous genetic mutations – hundreds of millions of these. Only a tiny number of them fixed in the populations and only 10 – 20  were identified as having positive effects on the populations.

Then Lenski’s team discovered something very unusual, as New Scientist explains in this 2008 article to mark the 20th year of the experiment:

Mostly, the patterns Lenski saw were similar in each separate population. All 12 evolved larger cells, for example, as well as faster growth rates on the glucose they were fed, and lower peak population densities.

“But sometime around the 31,500th generation, something dramatic happened in just one of the populations – the bacteria suddenly acquired the ability to metabolise citrate, a second nutrient in their culture medium that E. coli normally cannot use.

“Indeed, the inability to use citrate is one of the traits by which bacteriologists distinguish E. coli from other species. The citrate-using mutants increased in population size and diversity.”

Dawkins in his speech last week described genetic mutation as an incredibly random incident that if successful for the organism, is followed by a fairly predictable path of evolution. But was the ability to metabolise citrate the result of one random genetic mutation?

New Scientist continues:

By this time, Lenski calculated, enough bacterial cells had lived and died that all simple mutations must already have occurred several times over.

That meant the “citrate-plus” trait must have been something special – either it was a single mutation of an unusually improbable sort, a rare chromosome inversion, say, or else gaining the ability to use citrate required the accumulation of several mutations in sequence.

Lenski had been freezing samples of the bacteria every 500th generation from the very beginning, so he was able to go back through the generations, revive the frozen bacteria and see if they would evolve as the same citrate-gobbling mutants. He was able to use genetic markers to show the experiments weren’t subject to contamination. Lenski found that cloned populations from those frozen samples were able to develop the ability to use citrate, put only in bacteria drawn from populations 20,000 generations old or greater and only very rarely (around once per trillion cells).

Some type of mutation must have happened around generation 20,000 the researchers suggest, that set the path for evolution that would be triggered with subsequent mutation around generation 31,000 – 31,500.

Wikipedia sums it up best:

The authors interpret these results as indicating that the evolution of citrate utilization in this one population depended on an earlier, perhaps non-adaptive potentiating mutation that had the effect of increasing the rate of mutation to citrate utilization to an accessible level (with the data they present further suggesting that citrate utilization required at least two mutations subsequent to this potentiating mutation). More generally the authors suggest that these results indicate (following the argument of Stephen Jay Gould) “that historical contingency can have a profound and lasting impact” on the course of evolution.[4]”

Absolutely staggering stuff and an experiment that is yet to be mined for still more gems of knowledge about evolution. Who knows what the future holds for those generations of bacteria, multiplying, mutating and evolving in their own little lifecycle in that lab in Michigan…


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