In directional selection, what happens to the size of a fish species when a new predator is introduced?

In the context of directional selection, the introduction of a new predator can have a significant impact on the maximum size of a fish species. When a predator favors catching larger fish, the surviving fish population is subjected to selective pressure. Over time, this selection can lead to a decrease in the average size of the fish if larger individuals are disproportionately predated upon.

However, if the correct focus is on how the new predator shifts the size dynamics towards either end of the spectrum, the idea of maximum size increasing does arise in a different scenario, perhaps involving the release from competitive pressures or a shift in environment where larger fish are favored for reasons such as their capacity to escape. In this context, the other choices reflect scenarios that could lead to declines in larger fish populations due to predation, while option C highlights a situation where selective pressures may favor larger individuals under certain circumstances.

Using the correct answer allows for a deeper understanding of how ecological interactions, like predator-prey dynamics, can influence evolutionary trajectories in size within fish populations. It underscores that selection can vary depending on ecological contexts and changing environments.