Goldenrod gall flies (Eurosta solidaginis) lay their eggs individually in developing shoots of the tall goldenrod plant (Salidago). When it hatches, the fly larva makes a chemical the causes the plant tissue to swell around it, forming a mass of plant tissue called a gall, which serves as a home for the larva.

Downy woodpeckers attack large galls and eat the gall fly larva. Parasitic wasps lay eggs inside small galls and the wasp larvae that hatch from the eggs eat the gall fly larva when it hatches. Only galls of intermediate size remain intact because the wall of the gall is too thick for the ovipositor of a parasitic wasp to penetrate, and the size of the gall and hence the larva within makes it less attractive to woodpeckers than a larger gall containing a larger larva and hence a larger meal.

Which graph best depicts the type of selection occurring in this gall fly population?

Question

Goldenrod gall flies (Eurosta solidaginis) lay their eggs individually in developing shoots of the tall goldenrod plant (Salidago). When it hatches, the fly larva makes a chemical the causes the plant tissue to swell around it, forming a mass of plant tissue called a gall, which serves as a home for the larva.

Downy woodpeckers attack large galls and eat the gall fly larva. Parasitic wasps lay eggs inside small galls and the wasp larvae that hatch from the eggs eat the gall fly larva when it hatches. Only galls of intermediate size remain intact because the wall of the gall is too thick for the ovipositor of a parasitic wasp to penetrate, and the size of the gall and hence the larva within makes it less attractive to woodpeckers than a larger gall containing a larger larva and hence a larger meal.

Which graph best depicts the type of selection occurring in this gall fly population?

BlackTom AI · Accepted Answer

To determine the type of selection acting on the gall fly population, I will evaluate how each option describes changes in the distribution of gall sizes.
Option 1 (the top graph) shows a population distribution that becomes taller and more centered around a middle size, with reduced variation on the tails. This pattern indicates stabilizing selection, where intermediate phenotypes (intermediate gall sizes) have higher fitness and extreme sizes are selected against. In this scenario, galls that are too small or too large are less likely to be represented in the next generation, resulting in a narrower distribution around the optimum.
Option 2 (the middle graph) displays a bimodal or valley-like pattern, with two distinct peaks and reduced frequency in the middle. This is characteristic of disruptive selection, where extreme phenotypes at both ends have higher fitness than the intermediate forms, leading to a split in the population rather than a single central peak.
Option 3 (the bottom graph) shows a shift in the entire distribution toward one end (a change in the mean) while maintaining a similar overall shape and spread. This corresponds to directional selection, where one extreme or one range of sizes becomes more common over time, moving the mean but not necessarily narrowing the variance.
Option 4 is an empty choice and provides no information to evaluate.
Putting this together with the ecological description—parasitism and predation pressure favoring intermediate gall sizes because extremes are more vulnerable or less fit—the stabilizing selection model best fits the scenario, as intermediate galls are most likely to persist while extreme sizes are selected against.
Therefore, Option 1 best depicts the type of selection occurring in the gall fly population.

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