Evolution works by the non-random selection of random mutation. Natural selection accumulates favourable random chance events.
The experiment that was began on 24th February 1988 on E coli bacteria by Dr Richard E. Lenski and his team is surely one of the clearest demonstrations of the power of this process.
E.Coli is one of the commonest bacterium on earth, there is around 100 billion, billion of them in the world at any given time and around 1 billion of them in your gut right now. Most of the time they cause no problem, until a new strain wreaks havoc on its host's digestive system.
If we assume the probability of a particular gene mutating to be 1 in a billion, the size of the population is so high that just about every gene in the E.coli genome will have mutated somewhere in the world every day. These bacteria reproduce asexually through simple cell division so Lensky began by cloning a population of genetically identical individuals. Next he divided them equally into 12 identical flasks each of which contained the same nutrient broth to produce 12 tribes of E.Coli, which have remained totally separate for 28 years and counting.
Every day 12 new flasks are prepared with the very same broth consisting of a mixture that contains glucose that they feed on, and citrate, which they are not able to eat, but more on that in a minute. Exactly 1% of the population of each flask is removed daily and put into the new flasks. That means that there are now 12 lines of flasks each stretching way back the beginning if the experiment.The 12 tribes were sampled at intervals to see how they were changing and, at strategic points, samples were frozen to provide living “fossils” that could be resuscitated for comparison with later generations.
E.Coli don’t waste any time in reproducing,averaging between 6 and 7 generations per day. As of April 2014 Lenski has bred 60,000 generations of bacteria. If we were to scale that up to human generations it would take us back around a million years - a relatively modest span in evolutionary terms.
Every day the same pattern was observed, the population of the lucky 1% would initially soar, then as the food began to run out, it would level off as starvation set in: Boom and bust, day after day for 60,000 generations. The question to be answered was whether or not the bacteria would evolve, and if so would they change in similar or different ways?
The expectation was that if a bacterium underwent a mutation that allowed it to make better use of the limited food supply then it would be favoured by natural selection and in time the mutant would take over the tribe. Well this is exactly what happened in all 12 tribes, they all got better than their ancestors at exploiting the available glucose. What was really amazing was that they all got better in different ways; they each discovered their own novel mutations to improve their fitness.
In each case populations began to grow faster and the average body size of the bacteria grew. Most of this growth happened over the first 2000 generations after which it began to plateau. The graphs of their growth all fit a hyperbolic curve beautifully, but each curve follows a slightly different path as different mutations occur at different times in each tribe.
The graphs also show an exception to this rule of diversity however. Two tribes appeared to follow identical rates of growth over 20,000 generations. Lenski and a team of scientists investigated by studying the DNA of the populations concerned. The astonishing result they discovered was that the same 59 genes had undergone the same changes in both tribes. This is truly staggering.
The genome of the E.Coli bacterium contains 4,403 genes made up from 4,639,221 base pairs. So what are the chances of the same mutation happening independently in 2 populations? Fairly low but not unreasonable. So what if we find 2, 3 or 4 mutations the same? Now it’s getting remarkable. But 59 changes in the same genes in both populations! Here is an event, or series of events, that defy stupefying odds that would beggar belief if it were not for the fact that it actually happened in the lab and the evidence is there for any competent scientist to examine.
This is the whole point about the power of natural selection, it achieves things that appear impossible through the step by step accumulation of favourable changes. Both tribes had independently discovered the same 59 mutations out of all the millions of possible changes.
At generation 33,000 something else happened that was utterly remarkable, something that strikes at the very heart of the “Intelligent Design” movement. As the population of bacteria in a flask grows the liquid becomes increasingly cloudy. Each day the “cloudiness” or optical density (OD) is carefully measured and recorded. The OD of one particular tribe, named Ara-3, had been coasting along at a level of 0.04 similar to the other 11, when it suddenly it went into vertical take-off growing six-fold to an OD level of 0.25. After a few days the population level stabilized and all future generations of this tribe, and this tribe alone, achieved the same results.
So what was going on? Remember I mentioned earlier that the broth contained citrate? Well it turned out that Ara-3 had evolved the ability to metabolise citrate and therefore had loads more food to eat than the other 11 tribes. So when the other tribes were beginning to starve after the glucose ran out this tribe was still enjoying a bonanza.
Lenski worked out that this change was not likely to be the result of a single mutation, as it should have been discovered by other tribes as well. Neither was it a series of mutations of the sort where each change builds on the previous. That would not be rare enough to account for the dramatic uniqueness of Ara-3. What was needed was a combination of mutations of the kind that creationists call “irreducible complexity” where 2 or more mutations are required before there is any advantage to the organism.
One of Lenski’s students, Zachary Blount, ran a gruelling set of experiments involving 40 trillion E. Coli cells from across the generations to discover what had actually happened. The magic moment turned out to be around generation 20,000. Thawed out clones dating from before that point never discovered how to use citrate, they were just like the other 11 tribes. Those that dated from after that generation showed increased probability of subsequently evolving citrate capability. A mutation had occurred that conferred no advantage the bacterium but it primed it to take advantage of a later mutation. Not only does this show new "information" entering genomes - something the likes of John Mackay endlessly asserts is impossible - it demonstrate the power of natural selection to put together combinations of genes that, by the naïve calculations so beloved of creationists, should be tantamount to impossible. It also undermines their central dogma of “irreducible complexity”.
Lenski's research shows, in microcosm and in the lab, massively speeded up so that it happened before our very eyes, many of the essential components of evolution by natural selection: random mutation followed by non-random natural selection; adaptation to the same environment by separate routes independently; the way successive mutations build on their predecessors to produce evolutionary change; the way some genes rely, for their effects, on the presence of other genes. Yet it all happened in a tiny fraction of the time evolution normally takes.
In 2006 Lenski was rewarded for his outstanding work with his election to the United States National Academy of Sciences. His experiment is still ongoing.
|Part 1 - Protein Functional Redundancy - - - - - - - - -||Part 2 - DNA Functional Redundancy|
|Part 3 - ERVs||Part 4 - Smelly Genes|
|Part 5 - Vitamin C||Part 6 - Human Chromosome 2|
|Part 7 - Human Egg Yolk Gene||Part 8 - Jumping Genes|
|Part 9 - Less Chewing More Thinking||Part 10 - Non-Coding DNA|
|Part 11 - Titktaalik|