High School BiologyEvolutionHS-LS4-4

Evolution in Action

Antibiotic and pesticide resistance and the changing beaks of Galápagos finches after a drought show natural selection measured within a few generations.

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Evolution usually takes many generations, but when selection is strong and generations are short, we can watch it. These cases put directional selection and populations changing over time on a timeline you can see.

Antibiotic resistance

When an antibiotic is used, it kills the bacteria it can. A few bacteria may already carry an allele that lets them survive. Those survivors multiply, so the next population is mostly resistant. The CDC describes resistance as germs developing "the ability to defeat the drugs designed to kill them," and notes that antibiotics do not work on viruses. Insects become resistant to pesticides the same way.

After the drug, a single resistant survivor that doubles each generation becomes 64 resistant bacteria in six generations.

Finch beaks in a drought

On the small Galápagos island of Daphne Major, Peter and Rosemary Grant measured medium ground finches year after year. In the 1977 drought, small soft seeds ran out and the population fell by about 85%. Birds with deeper beaks could crack the hard seeds that were left, so more of them survived. Their offspring also had deeper beaks on average: evolution measured in one generation.

Key idea

In each case the variation was already there. The drug, the pesticide or the drought did not create the helpful trait. It selected among traits that already varied, and the next generation inherited the survivors' alleles.

Why it works

A mutation is a copying change, and copying changes happen whether or not they would help. The environment cannot reach into the DNA and order the right one. What the environment does is decide who leaves offspring. That is why resistance spreads faster when a drug is used more: more use means stronger selection.

Worked example: Counting survivors

A colony of 1,000,000 bacteria has 10 resistant cells. An antibiotic kills every other cell. The survivors double every hour. How many bacteria are there 5 hours later?
  1. Survivors: the 10 resistant cells.
  2. Five doublings multiply by $2^5 = 32$.
  3. 10 × 32 = 320.

Answer: 320 bacteria, all resistant.

Common mistake

A very common idea is that the drug or the drought caused the needed change, as if the bacteria "learned" to resist. The variation was there first; the environment only chose among it. A quick check: in the finch study, the deep beaks were measured before the drought began.

Practice

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Sources and further reading

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