Final answer:
The observation of the average head size in a population of toad-eating snakes getting smaller is an example of directional selection.
Step-by-step explanation:
This is an example of directional selection. Directional selection occurs when individuals with a trait that is beneficial in a particular environment have a higher fitness and are more likely to survive and reproduce. In this case, the toad-eating snakes with smaller heads are unable to eat the large cane toads, so they have a lower fitness and are less likely to pass on their genes compared to snakes with larger heads. Over time, this can lead to a decrease in the average head size in the population.
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