A population of squirrels shows a range of sizes, with intermediate-sized animals selected for. What type of selection does this represent?

The scenario described illustrates stabilizing selection, which is a type of natural selection that favors intermediate phenotypes within a population. In this case, the intermediate-sized squirrels are being selected for, meaning that individuals that are either larger or smaller than the average size are less likely to survive or reproduce successfully. This leads to a decrease in variance and maintenance of the population's average size over time.

Stabilizing selection often occurs in stable environments where extreme traits are not advantageous and can lead to increased survival and reproductive success for those individuals that are more typical in their size or other traits. This process helps reinforce the advantages of the average phenotype, which may be better suited for the challenges of their habitat, such as foraging, avoiding predators, or competing for resources.

In contrast, directional selection would favor one extreme trait over another, leading to a shift in the population's phenotype over time, while disruptive selection would favor both extremes and can potentially lead to the formation of two distinct groups within a population. Artificial selection involves human intervention to breed individuals for certain traits, which is not indicated in this natural context.