The answer is natural death due to old age. Of the typical answer choices presented in this question, chipmunks dying of old age is the least likely to shift the gene pool because old age removes individuals more or less at random with respect to their genetic makeup. By the time a chipmunk dies of old age, it has already had the opportunity to reproduce and pass its alleles to the next generation. No specific set of genes is being targeted or filtered out.
The other options, a pesticide killing many chipmunks, a group migrating to a new habitat, or a mutation producing larger eyes, all introduce mechanisms that actively change which alleles are represented in the population. Understanding why requires a closer look at what actually shifts a gene pool.
Why Old Age Doesn’t Filter Genes
A gene pool changes when certain alleles become more or less common across generations. For that to happen, something has to favor or remove specific genetic variants before they get passed on. Death from old age doesn’t do this. A chipmunk that lives a full lifespan has already mated and contributed offspring. Its death removes it from the living population but not from the genetic record of the next generation. Because old age doesn’t preferentially strike chipmunks carrying particular alleles, it has no directional effect on allele frequencies.
Compare this with a pesticide that wipes out a large portion of the population. Some chipmunks may have slight genetic differences in metabolism or behavior that make them more likely to encounter or survive the poison. The survivors carry a non-random subset of the original gene pool into the next generation, permanently altering allele frequencies. That’s natural selection in action.
How the Other Options Change the Gene Pool
Pesticide Killing Many Chipmunks
A mass die-off from a pesticide acts like a strong selection event. Even if the pesticide isn’t targeting a specific trait, drastically reducing population size triggers genetic drift, where random chance alone can eliminate alleles simply because so few individuals survive. The smaller the surviving group, the faster drift reshapes the gene pool. A population bottlenecked from hundreds to a few dozen can lose rare alleles in a single generation.
Migration to a New Habitat
When a group of chipmunks leaves and establishes itself elsewhere, two things happen. The original population loses whatever alleles those emigrants carried, and the new population starts with only the genetic diversity of its founders. This is gene flow in reverse: alleles are subtracted from one pool and concentrated in another. Research on chipmunk species has shown that gene flow between populations can either homogenize genetic differences or, when it stops, accelerate divergence between groups. Either way, the gene pool changes.
A Mutation Producing Larger Eyes
A new genetic mutation is, by definition, a new allele entering the gene pool. Even if it starts in a single chipmunk, it has the potential to spread through the population over generations, especially if it confers any survival or reproductive advantage. Studies on eastern chipmunks have found that even behavioral traits like exploration tendency face strong selection pressures: individuals with either very low or very high exploration scores were almost twice as likely to survive a six-month period compared to those in the middle range. A visible physical change like larger eyes would almost certainly face similar selective forces, amplifying the mutation’s impact on the gene pool over time.
The Hardy-Weinberg Framework
Biologists use a set of conditions called Hardy-Weinberg equilibrium to describe when a gene pool stays perfectly stable. Five things must all be true simultaneously: no mutations occur, no individuals migrate in or out, mating is completely random, the population is large enough that chance fluctuations are negligible, and no natural selection is operating. In the real world, no population meets all five conditions, which is why evolution is constant. But this framework is useful for identifying which forces are responsible for change.
Each of the “wrong” answer choices in this question maps directly onto a violation of these conditions. A pesticide introduces natural selection and potentially genetic drift. Migration violates the no-gene-flow requirement. A mutation violates the no-new-alleles requirement. Death from old age, on the other hand, doesn’t cleanly violate any of these conditions. It doesn’t select for or against particular traits, it doesn’t move alleles between populations, and it doesn’t create new genetic variants. It’s simply the endpoint of a life that has already contributed to the gene pool.
Why This Question Matters Beyond the Exam
This question tests whether you can distinguish between events that look dramatic and events that actually alter genetic composition. A lot of chipmunks dying sounds like it should change the gene pool, and it can, but only if the deaths are non-random with respect to genetics. Old age is about as close to genetically neutral as a cause of death gets. The key insight is that timing matters: if an organism has already reproduced, its death has far less genetic consequence than the death of a young organism that never got the chance to pass on its alleles.
This same logic applies across species. In conservation biology, the concern with endangered populations isn’t that individuals die, it’s that they die before reproducing, or that so few survive a catastrophic event that the remaining gene pool is too narrow to sustain healthy genetic diversity. The mechanism of death matters far more than the fact of it.