Lenski experiment increase in fitness

The smooth curve in the graph of increasing cell size as the generations go by gives support to the idea that the improvement is gradual. But perhaps it is too gradual? Wouldn't you expect to see actual steps, as the population 'waits' for the next improving mutation to turn up? Not necessarily. It depends on factors such as the number of mutations involved, the magnitude of each mutation's effect, the variation in cell size that is caused by influences other than genes, and how often the bacteria were sampled. And interestingly, if we look at the graph of the increase in fitness, as opposed to cell size, we do see what could at least be interpreted as a more overtly stepped picture (above). You remember, when I introduced the hyperbola, I said it might be possible to find a more complicated mathematical function that would fit the data better. Mathematicians call it a 'model'. You could fit a hyperbolic model to these points, as in the previous graph, but you get an even better fit with a 'step model', as used in this picture. It is not such a close fit as the cell size graph's fit to a hyperbola. In neither case can it be proved that the data exactly fit the model, nor can that ever be done. But the data are at least compatible with the idea that the evolutionary change that we observe represents the stepwise accumulation of mutations.*

We have so far seen a beautiful demonstration of evolution in action: evolution before our very eyes, documented by comparing twelve independent lines, and also by comparing each line with 'living fossils', which literally, instead of only metaphorically, come from the past.

Now we are ready to move on to an even more interesting result. So far, I've implied that all twelve tribes evolved their improved fitness in the same general kind of way, differing only in detail - some being a bit faster, some a bit slower than others. However, the long-term experiment threw up one dramatic exception. Shortly after generation 33,000 something utterly remarkable happened. One out of the twelve lineages, called Ara-3, suddenly went berserk. Look at the graph opposite. The vertical axis, labelled OD, which stands for optical density or 'cloudiness', is a measure of population size in the flask. The liquid becomes cloudy because of the sheer numbers of bacteria; the thickness of the cloud can be measured as a number, and that number is our index of population density. You can see that up to about generation 33,000, the average population density of Tribe Ara-3 was coasting along at an OD of about 0.04, which was not very different from all the other tribes. Then, just after generation 33,100, the OD score of Tribe Ara-3 (and of that tribe alone among the twelve) went into vertical take-off. It shot up sixfold, to an OD value of about 0.25. The populations of successive flasks of this tribe soared. After only a few days the typical plateau at which flasks of this tribe stabilized had an OD number about six times greater than it had been, and than the other tribes were still showing. This higher plateau was then reached in all subsequent generations, in this tribe but no other. It was as though a large dose of extra glucose had been injected into every flask of Tribe Ara-3, but given to no other tribe. But that didn't happen. The same glucose ration was scrupulously administered to all the flasks equally.

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