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Evolution in Action - Visualizing Bacteria Evolving Antibiotic Resistance

e. coliNot too long ago, I came across a question on Quora, Evolutionary biologists usually say that organisms adapt to their environment. Does this not contradict Darwinism?. It seemed like a good opportunity to explain how natural selection adapts organisms to their environments, and especially to use a recent experiment involving e. coli. Here's what I wrote, with some very minor edits.


I'm going to assume that by 'Darwinism', you mean natural selection. Organisms adapting to their environments is pretty much textbook natural selection, but let's go through an example to see what this means.

There was a very interesting experiment/demonstration last year involving bacteria and antibiotics. A team of researchers from Harvard Medical School and Technion - Israel Institute of Technology made in effect a giant petri dish - a rectangle 2 ft x 4 ft. The unique aspect of this petri dish, besides its size, was that it was divided into regions with varying concentrations of an antibiotic. Either end of the rectangle was free of antibiotic. The next region in had the minimum concentration to kill the e. coli bacteria that were the subject of the experiment. Each subsequent region moving in increased the concentration ten fold, until the center region, which had a concentration 1000 times higher than what would normally kill e. coli.

e. coli experiment setup
Image Source: Screen Capture from Video Shown Below
(Click to embiggen)

So, the researchers seeded the antibiotic free ends with e. coli, and then let them grow, taking periodic photos of the petri dish, and combinging them all into a time lapse movie. I'd really recommend watching the whole thing. It's really very interesting, with more explanation than what I've provided here, and only 2 minutes long.

So, let's take a closer look at one instant to see what exactly is going on. At one point, the tray looked like this:

e. coli experiment screenshot 1
Image Source: Screen Capture from Video Shown Above
(Click to embiggen)

So, the antibiotic free ends are completely colonized by bacteria. The two regions with the lowest concentration of antibiotic have just begun to be colonized. There are several small resistant colonies, and you can see where each one of those colonies got their starts. What happened was that the original e. coli, with no antibiotic resistance spread across the agar until they hit the antibiotic. Since they weren't resistant, that was as far as they could go without dying. But those e. coli kept on living and reproducing, with mutations appearing throughout the population. In bacteria that just happened to be at the boundary of the antibiotic, who also happened to acquire just the right mutations to make them resistant to the antibiotic, they now had a whole new environment opened up to them and their descendants.

Notice that there's really no pattern to where those colonies got their starts. It was basically random, because mutations are random. No bacteria were trying to evolve. No bacteria were attempting to figure out a strategy to survive the antibiotic. Bacteria don't even have brains to try to do any of that. It was just whatever bacteria happened to be lucky enough to acquire the appropriate mutations by chance, an error at the chemical level when copying DNA.

Once those first resistant bacteria entered this new region, they spread. Then, once they hit the region with 10x the antibiotic concentration, they were contained again, until a few more bacteria happened to acquire the proper mutations by luck, and had a new environment opened up to them and their descendants. This repeated, until the bacteria were eventually colonizing the region with 1000x the concentration of antibiotic that would have killed the original e. coli that seeded the plate:

e. coli experiment screenshot 2
Image Source: Screen Capture from Video Shown Above
(Click to embiggen)

So, these e. coli were adapting to their environment. However, it wasn't any conscious intent, or Lamarckian type of use and disuse. It was random mutations creating variation in the e. coli populations. Whichever e. coli happened to be lucky enough to have mutations to survive the antibiotic were the ones that thrived. Any e. coli that weren't lucky enough to have those mutations were limited to their existing environments.

This experiment had a pretty strong selection pressure with the antibiotic, but the same principles are at work in nature with other selection pressures. Whatever individuals happen to be lucky enough to acquire by chance the mutations best suited to an environment will be the ones that have the most offspring, increasing the frequency of whatever mutation that benefited them. Multiply this over generations, with natural selection 'ratcheting' additional mutations, so that the population becomes better and better suited to the environment. That is what is meant by saying that organisms adapt to their environment.


More info on the e. coli experiment:

Image source: Wikimedia Commons

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