In a forest, a tree falls on a swamp and kills the worm population reducing the population by half. The remaining population is characterized by a remarkable reduction in genetic diversity. Which mechanism of evolution is exhibited here?

The answer is genetic drift (bottleneck effect)

There are 4 basic mechanisms of evolution - mutation, genetic drift, natural selection, and gene flow (migration). Genetic drift is a change in the frequency of alleles in a population as a result of random sampling of organisms. One of the examples of genetic drift is the bottleneck event. The bottleneck event means drastic reduction of the population size because of environment changes. The consequence of this is the reduction of genetic variation. This is what happened to the population of worms after tree falling.