Final answer:
The decrease in the average size of guppies when a predator was introduced demonstrates natural selection, as smaller guppies likely had a survival advantage, leading to an increased frequency of alleles for smaller size.
Step-by-step explanation:
The evolution mechanism demonstrated when scientists observed that the average size of guppies in the pond decreased over a few years after a guppy predator was introduced is natural selection. This is because the presence of predators likely favored smaller guppies that could evade detection more easily than larger ones. The smaller size trait, therefore, became more prevalent in subsequent generations.
Gene flow refers to the movement of alleles between populations, genetic drift describes how allele frequencies can change by chance events, and mutations are changes in the DNA sequence that can create new genetic variation. In the case of the guppies, the directed pressure of predation is the cause for the change in allele frequency favoring smaller size, which is a clear example of natural selection.