If the frequency of a particular allele that is present in a small, isolated population of alpine plants decreases due to a landslide that leaves an even smaller remnant of surviving plants bearing this allele, then what has occurred?

Evolution

Explanation:

A bottleneck genetic drift and micro-evolution have occurred in the given condition

In bottleneck effect, inferior genotype is eliminated from a homogeneous gene pool due to strong natural selection pressure and changing environmental conditions and this leads to genetic drift as a result only a few most adapted genotype survives and flourish Micro-evolution is a type of evolution in which formation of new species occurs at a slow pace following phyletic gradualism Causes of micro-evolution are point mutation and recombination

Related dating is the answer