Fossil evidence indicates that horses have gradually increased in size over geologic time. Which of the following terms best describes this?

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Directional selection is a type of natural selection that occurs when individuals with traits at one end of the spectrum are more successful in their environment, leading to a shift in the average phenotype of a population over time. In this case, horses that were larger may have had an advantage in surviving and reproducing, causing the average size of horses to gradually increase over time.

Fossil Evidence Indicates That Horses Have Gradually Increased in Size