Your friend has a dog with fleas. She goes to the pet store and buys over-the-counter flea medicine. It does not work. So she goes to the veterinarian and explains that she just spent $80 on medicine that does not work. The vet casually says, “Oh yeah, that medicine does not work anymore.” Because you now understand natural selection, your friend asks you questions. Answer them with evidence.

- What do the resistant fleas do that the sensitive fleas do not?

Explanation:

The dog has fleas, but the medicine to counter this does not work. This means that the fleas have built up a resistance against the drug to ensure continued survival and reproductive success of these organisms. These resistant fleas may have existed in population as the population is always large but in small amounts. Thus, treatment with the drug will lead to the dieing out of the sensitive fleas and then leaving the resistant fleas with the favourable alleles to be selected for.

These favourable traits that have evolved against the drugs are then selected for and the organisms reproduce giving rise to more fleas with these favourable characteristics (being resistant to the drugs) such that their continued survival and reproductive is ensured compared to the sensitive fleas that die out.

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