The gall fly Eurosta solidaginis lays its eggs in the buds of the goldenrod plant. Once hatched, the larvae produce a gall, in which they develop inside. Smaller galls are subjected to higher levels of parasitism by a species of parasitoid wasp, while larger galls are subjected to higher levels of predation by birds such as the Downy woodpecker. The type of selection the galls are under is best described as:

Question

BlackTom AI · Accepted Answer

Question recap: This scenario discusses gall size in Eurosta solidaginis and how mortality pressure varies with gall size, leading to a description of the type of selection acting on gall size.
Option a: stabilising. This choice is consistent with stabilising (stabilizing) selection, where intermediate phenotypes have higher fitness and extremes suffer higher mortality. Here, small galls face high parasitism and large galls face high predation, while mid-sized galls experience relatively lower overall mortality, pushing the population toward an intermediate optimum.
Option b: disruptive. Disruptive selection favors extreme phenotypes over intermediates, leading to two or more peaks in the distribution. The described pattern shows higher mortality at both ends (small and large) and lower mortality in the middle, which is the opposite of disruptive selection, so this option is incorrect.
Option c: negative frequency-dependent. Negative frequency-dependent selection benefits rarer phenotypes, so the fitness of a gall size would increase when that size is common to a greater extent than when it is rare. The scenario ties fitness to size-related mortality rather than to the relative frequency of gall sizes, so this option does not fit the description.
Option d: directional. Directional selection shifts the mean phenotype in one direction (either toward larger or smaller gall sizes) due to consistent selection pressure in one direction. The mortality pattern here does not show a shift in one direction but rather higher mortality at both extremes and lower at intermediates, which argues against directional selection.
Summary: The observed pattern—lower mortality at intermediate gall sizes and higher mortality at both small and large sizes due to different predators/parasitoids—fits stabilising selection, making option a the correct description of the selective regime affecting gall size.

BIO3020 - Evolutionary Ecology - S1 2025

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