The fluctuation test performed by Luria and Delbrück is consistent with the random mutation hypothesis. Briefly describe their experiment and identify how the results match the prediction of the random mutation hypothesis. What would have to be different about the experimental results for them to agree with the prediction of the adaptive mutation hypothesis?

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Hi, everybody. Welcome to our next problem. If Luria and Del Brook's FLT test had supported the adaptive mutation hypothesis. What would the distribution of mutants among cultures have looked like? Choice? A to A says it would have followed a Poisson distribution to B. It would have been biased towards specific stress conditions. Choice C it would have been lower in cultures with fewer bacterial cells or choice D it would have been the same in all cultures. So let's recall what the adaptive mutation hypothesis is. And that says, but organisms can produce mutations specifically tailored to help them survive in challenging conditions. So it's assuming that more mutations will arise in stressful environments. So this idea that being in a stressful environment in and of itself causes organisms to have more mutations since there's a greater need for adaptive mutations. So in that case, the fluctuation test which looked at mutations to bacteria under challenging and normal conditions. So the fluctuation test. So in the case of that particular test, it was bacteria grown in the absence or the presence of a bacteria phage and looking for mutants that had a resistance to that age. And therefore, weren't killed by infection. If the adaptive mutation hypothesis were true, you'd expect to see more bacterial colonies growing when exposed later to the phage because you'd have expected to see more mutations arise in bacteria that had earlier been exposed to the phage. A stressful condition versus bacteria that have been grown without exposure. However, the results of this test showed that the same number of mutations occurred no matter the conditions. So if that test had supported the adaptive mutation hypothesis, then you would have expected to see choice B, it would have been biased towards specific stress conditions because you would have seen that under stress, you would have seen more mutations arise. Let's just look at our other answer. Choices to understand why they're not correct. Choice. A it would have followed a Poisson distribution. This is a statistical distribution looking at how often certain events occur and looking at their distribution over time, but this wouldn't have proven an adaptive mutation hypothesis. So that's not correct. It could have been, you could have mapped it out in a Poisson distribution, but that isn't what would have proved the truth of the adaptive mutation hypothesis. So I see it would have been lower in cultures with fewer bacterial cells. Well, the number of bacterial cells would not be relevant. In this case, you wouldn't expect to see more or less mutations depending on the number of cells you started with. So choice C is not correct and then finally, to d it would have been the same in all cultures. And of course, that's what we did see. And so that showed that the adaptive mutation hypothesis was wrong because you saw the same number of mutations under stress or under normal conditions. So again, if that fluctuation test had supported the adaptive mutation hypothesis, what would that distribution have been choice B bias towards specific stress conditions? See you in the next video.

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Key Concepts

Random Mutation Hypothesis

Luria and Delbrück Experiment

Adaptive Mutation Hypothesis