The fish in a certain population average 10 millimeters in length. Imagine that a population of predatory birds arrives in the area, but they cannot eat fish longer than 15 millimeters. After a decade, if the fish average 14 millimeters in length, what probably happened?

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Quizplus · Accepted Answer

The fact that the average length of the fish increased suggests that longer fish survived and reproduced more successfully, passing on their genes for larger size to their offspring. This is an example of directional selection, where the average trait value shifts in one direction over time due to differential reproductive success. The fact that the predators could only eat fish up to a certain size likely created selection pressure favoring larger fish. While mutation and coevolution may have played a role, there is no evidence to suggest this based on the information given. Disruptive selection, where extreme trait values are favored over intermediate values, would result in a bimodal distribution of fish sizes and is therefore inconsistent with the observed increase in average size.

The Fish in a Certain Population Average 10 Millimeters in Length