The graph below shows a fitness curve based on body size for a fictional population of butterflies. Assuming all the necessary and sufficient conditions for selection to occur are met in this population, what pattern of selection does this fitness curve predict will occur?

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BlackTom AI · Accepted Answer

To begin, let's restate the core elements of the question: we have a graph of fitness (mortality) versus body size in butterflies, and we assume the standard conditions for selection hold. The key pattern to infer is how fitness changes with body size.
Option 1: Stabilizing selection. In stabilizing selection, individuals with intermediate trait values have higher fitness than those at the extremes. Looking at the graph, mortality is lowest at intermediate body sizes and higher at both very small and very large sizes. Since higher fitness corresponds to lower mortality, this pattern supports stabilizing selection, where the population mean is favored and variation around it is selected against.
Option 2: Directional selection in favor of smaller body size. This would require a fitness curve that increases consistently as body size decreases (or mortality decreases as size increases in one direction). The depicted U-shaped curve shows higher mortality at both extremes, not a consistent advantage to smaller sizes alone, so this option doesn’t fit.
Option 3: Directional selection in favor of larger body size. Similar reasoning as above: directional selection would show fitness increasing in one direction (e.g., larger size lead to higher fitness). The graph does not show a linear increase with size; instead, the extremes are both disfavored, which contradicts a unidirectional shift.
Option 4: Diversifying/disruptive selection. This occurs when extreme trait values have higher fitness than intermediate values, leading to a bimodal distribution. In the graph, the intermediate sizes have the lowest mortality (highest fitness), while extremes have higher mortality, which is the opposite of disruptive selection.
Across these interpretations, the pattern in the graph precisely matches stabilizing selection: intermediate body sizes are favored, and both very small and very large sizes are selected against, leading to reduced variation around the mean.

BIS 002B Sections A and B SQ 2025 Practice Midterm #2

The graph below shows a fitness curve based on body size for a fictional population of butterflies. Assuming all the necessary and sufficient conditions for selection to occur are met in this population, what pattern of selection does this fitness curve predict will occur?

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