AP Biology · Unit 7 · Topic 7.8
Evolution is not something that finished. A flask of bacteria will show you the whole process in a week, and the experiment that proved it can be run here.
Learning objective 7.8.A Explain how evolution is an ongoing process in all living organisms.
Two explanations, and they are not easy to tell apart. Either the antibiotic makes bacteria resistant, or resistant bacteria were already there and the antibiotic killed the rest. In 1943 Luria and Delbrück worked out how to decide. Each bar below is one culture grown from a single cell, then plated on antibiotic; its height is how many colonies survived.
| Hypothesis | Mean | Variance | Variance ÷ mean | Cultures with none | Most in one culture |
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A culture starts as one cell and doubles twenty times. If a mutation happens at the first division, half the final culture is resistant. If it happens at the last division, one cell in a million is. Nothing about the mutation differs — only when it happened — and the clone has been doubling ever since.
That is a mechanism for producing enormous differences between flasks that were treated identically. Induction has no such mechanism: every cell meets the antibiotic at the same moment and gets the same independent chance, so the counts cluster around their average the way any independent sampling does.
The first tab established that the variation is there before the drug is. This one is what the drug does with it. One in a million cells is resistant at the start; watch what changes and what does not.
| Commonly said | What is actually happening | The evidence |
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Antibiotics, insecticides, herbicides, chemotherapy. Different organisms, different chemicals, different industries — and the same three conditions from Topic 7.1 being met each time.
Resistance is the visible case. The quieter one is that every genome is changing all the time, and the arithmetic is worth doing once.
| Quantity | Value |
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The influenza vaccine is remade annually not because the old one was badly designed but because the virus population it was designed against no longer exists in that form. A vaccine is a selective pressure like any other: the strains it fails to stop are the ones that spread.
| Requirement | In a bacterial infection | In a tumor |
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Answer these in your own words before you open the answers.
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