Resistance in Two Valleys: Selection on Variation That Was Already There
Alfalfa growers in two neighboring valleys farm the same aphid species. In Valley A, fields have been sprayed every June since 2015 with the same pyrethroid insecticide. Valley B has never been sprayed. Each year researchers sample aphids from unsprayed border strips and measure the frequency of a resistance allele, R. Valley A: 0.03 in 2015, 0.34 in 2018, 0.68 in 2021, 0.87 in 2024. Valley B: 0.02, 0.02, 0.03, 0.02 across the same years. A freezer collection gathered in Valley A in 2014, before the first spray was ever applied, shows R at 0.03. In laboratory cages with no insecticide, females carrying two copies of R lay about 15% fewer eggs than females carrying none. One copy of R is enough to survive a spray.
1. A grower says, "The spraying taught the aphids to resist - they switched the gene on when we started spraying." Use the 2014 freezer sample and the two-valley comparison to evaluate this claim.
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The claim is wrong in its mechanism, though right that spraying drove the change.
The 2014 sample: R was already present at 0.03 before any pyrethroid touched those fields. The allele existed in the population before the insecticide did, so no individual needed to acquire or activate anything. What the spray changed was not the supply of the allele but its value: susceptible aphids died, and aphids carrying R survived and reproduced. The frequency rose from 0.03 to 0.87 over nine years because of differential survival and reproduction, which is natural selection.
The two-valley comparison is the control that makes this argument strong. Valley B, genetically the same species and exposed to the same weather, stayed at 0.02–0.03 with no insecticide. If resistance were some general response to farming or a spontaneous trend, Valley B would show it too. It does not, so the spray is the selective agent.
Why the phrasing matters: mutations are not directed by need; they arise at random, and selection only sorts what already exists. The population adapted because variation was present, not because it was instructed.
2. Explain why the frequency in Valley B stayed near 0.02 for a decade, and why it did not drift down to zero.
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Why it stays low: with no insecticide, carrying R confers no advantage, and carrying two copies carries a real cost - about 15% fewer eggs. Selection therefore works mildly against R in Valley B, which keeps the frequency from climbing. Valley A shows the mirror image: the same allele is costly in the absence of spray and strongly favored in its presence, so the same starting frequency produced opposite trajectories depending on the environment.
Why it does not fall to zero: several forces keep replenishing R. New copies arise by mutation, though rarely. Immigration of aphids from Valley A on wind, equipment, or plant material introduces R at a meaningful rate once Valley A's frequency is high. And at a frequency near 0.02, almost all R copies sit in heterozygotes, which pay little or none of the fecundity cost - so selection against R is weak precisely because R is rare. Random drift at small population sizes also matters. Zero is not the expected endpoint; a low equilibrium maintained by mutation, migration, and the shielding effect of dominance is.
3. Suppose Valley A stops spraying entirely for the next eight years. Predict the direction of change in R frequency, give the mechanism, and explain why the decline would probably be slower than the rise was.
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Direction: R will decline from 0.87 toward the historical low of about 0.03, though the population will not become fully susceptible on a useful timescale.
Mechanism: the selective pressure reverses. With no pyrethroid, the 15% fecundity cost of carrying two copies now matters, so aphids without R leave more offspring and R becomes less common each generation. This is selection, not reversion - the allele is not being "switched off," it is being out-reproduced.
Why the fall is slower than the climb: when R is very common, most R copies sit in heterozygotes alongside a susceptible allele. If the cost falls mainly on females with two copies of R, much of the population pays little cost, and selection against R is weak. The rise, by contrast, happened when R was rare and spraying killed essentially every aphid without it - an enormous selective advantage. The same dominance that sped the increase now slows the decrease.
Practical consequence: eight spray-free years would probably lower the frequency but not restore full susceptibility, so the grower should plan to rotate among chemical classes, keep unsprayed refuges, and monitor rather than assume resistance has disappeared.