Pesticide resistance is the evolution of a pest population that can survive a chemical that used to kill it, caused by natural selection favoring individuals whose pre-existing genetic variation lets them survive and reproduce. It's a textbook AP Bio example of ongoing evolution (Topic 7.8).
Pesticide resistance happens when a population of insects (or weeds, fungi, etc.) evolves the ability to survive a chemical that used to wipe it out. Here's the key thing the AP exam wants you to get: the pesticide does NOT create resistance. The variation was already there.
In any large population, a few individuals carry random mutations that happen to let them survive the chemical. When the field gets sprayed, the susceptible ones die and the resistant ones live. Those survivors reproduce and pass their resistance alleles to the next generation. Spray again year after year, and the resistant allele keeps climbing in frequency until most of the population can shrug off the pesticide. That's just natural selection with a chemical doing the selecting.
This sits in Unit 7: Natural Selection, specifically Topic 7.8 Continuing Evolution. It's the headline example for learning objective AP Bio 7.8.A, which asks you to explain that evolution is an ongoing process in all living organisms. EK 7.8.A.1 lists resistance to antibiotics, pesticides, herbicides, and chemotherapy drugs as direct evidence that species are still evolving right now, not just in the deep past.
Why the College Board loves it: it's observable, it happens fast, and it forces you to apply the full natural-selection mechanism (variation, selection pressure, differential reproduction, allele frequency change) to a real situation instead of reciting a definition.
Keep studying AP® Biology Unit 7
Mutation and Genetic Variation (Unit 7)
Resistance has to start somewhere, and that source is random mutation. Mutation creates the genetic variation, and the pesticide simply selects from variation that already existed before the spray ever hit the field.
Antibiotic Resistance (Unit 7)
Pesticide resistance and antibiotic resistance are the exact same story with different actors. In one, a chemical selects for resistant insects; in the other, an antibiotic selects for resistant bacteria. Learn one mechanism and you've learned both.
Fossil Record (Unit 7)
EK 7.8.A.1 groups pesticide resistance with the fossil record as two kinds of evidence for ongoing evolution. The fossil record shows change across millions of years; resistance shows the same process happening in a single human lifetime.
Reproduction (Unit 7)
Surviving a pesticide doesn't matter for evolution unless survivors reproduce. Differential reproductive success is what actually shifts allele frequencies, so resistance spreads only because resistant individuals leave more offspring.
Expect this as a multiple-choice example of natural selection, often dressed up as a scenario or a data figure. A classic stem: a farmer sprays the same insecticide every year, and after several seasons the insect population bounces back despite continued spraying. You explain that the pesticide selected for pre-existing resistant individuals who then reproduced. You may also see a bar graph of LD50 (the dose lethal to 50% of a population) rising over years, and you'd interpret the increasing LD50 as the population evolving resistance. Some items give you flowcharts of the mechanism, and you pick the one that runs variation, then selection, then differential reproduction in the correct order. No released free-response question uses the phrase verbatim, but it's perfect support for any FRQ asking you to explain or provide evidence that evolution is ongoing.
An individual insect does NOT become resistant during its lifetime by being exposed. Resistance evolves at the POPULATION level across generations. The resistant insects were born resistant; the population just shifts toward them as the susceptible ones die off and the survivors breed.
Pesticide resistance is natural selection with a chemical as the selection pressure, and it's a core AP Bio example of ongoing evolution under Topic 7.8.
The pesticide does not create resistance; it selects for resistant individuals whose mutations already existed in the population.
Resistance evolves in populations across generations, not in individuals during their lifetimes.
A rising LD50 over time is data showing a population is evolving resistance, since it takes a bigger dose to kill the same fraction.
Pesticide, antibiotic, herbicide, and chemotherapy-drug resistance all run on the identical mechanism, so master one and you've got them all (EK 7.8.A.1).
It's when a pest population evolves the ability to survive a pesticide that used to kill it, through natural selection acting on pre-existing genetic variation. It's the go-to example in Topic 7.8 for proving evolution is still happening today.
No. The pesticide doesn't create resistant insects, it just kills the ones that lack a resistance allele. The resistant individuals were already there thanks to earlier random mutations, and spraying lets them dominate the next generation.
They're the same evolutionary process with different players. Pesticide resistance involves insects or weeds surviving a chemical spray, while antibiotic resistance involves bacteria surviving a drug. EK 7.8.A.1 lists both as evidence of ongoing evolution.
The spray kills the susceptible insects but spares the few resistant ones, which then reproduce. Year after year the resistance allele climbs in frequency until most of the population survives the pesticide, so the population rebounds.
An LD50 that increases over time (say from 10 units in 1995 to 50 units in 2015) means it takes more pesticide to kill half the population, which is direct evidence the population has evolved resistance.
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