243.

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		1.5.10— Example Where the Variance Does Not Exist: Evolution

		In the beginning of the 20th century, it was not known whether: (1) mutations occur all the time but are only selected when there is a change in the environment or (2) mutations occur only in response to a change in the environment.

		Luria and Delbruck let a cell multiply many times and then challenged its descendants with a killer virus. They ran many such experiments and determined how many cells were resistant to the virus at the end of each experiment.

		If mutations occur all the time, then by chance, some cells will become resistant to the killer virus before it is given. This resistant cell will divide and give rise to resistant daughter cells in subsequent generations. If the resistant cell is produced early on, it will form many resistant daughters in the many subsequent generations. If the resistant cell is produced later on, it will not have time to form as many resistant daughters. Each time the experiment is run the mutations will occur at different times. The variations in the timing of the appearance of the resistant mutations are amplified by the number of resistant daughters they produce in the subsequent generations. This results in a large variation in the final number of resistant cells when the experiment is repeated many times.

		If the mutations occur only in response to the virus, then by chance, some cells will become resistant to the killer virus when it is given. However, in this case, they will not have time to form resistant daughters. Thus there will be only a small variation in the final number of resistant cells when the experiment is repeated many times.

		Luria and Delbruck found that there was a large variation in the number of resistant cells. Thus they concluded that random mutations occur all the time, but are only selected when there is a change in the environment.

 

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