In a population of beetles, there are green and brown colorations that occur with equal frequency. after a forest fire, only 20 beetles remain. 18 of them are green and 2 of them are brown. what describes this change in frequencies?

A population bottleneck occurred, altering the resulting allele frequencies.

Explanation:

A population bottleneck occurs when the size of a population is reduced by at least a generation. In this case, genetic drift acts more quickly to reduce genetic variation in small populations, but going through a “bottleneck” can greatly reduce variation, even if the bottleneck does not last for many generations. This effect called a bottleneck usually happens when a major disaster occurs in the population's habitat, changing the allelic frequencies and even the genetic variability of the population.

The population bottleneck can be seen in the question above, where after a forest fire, only 20 beetles remain in a population of beetles. 18 of them are green and 2 of them are brown.