Genetic Drift is the change of certain gene frequencies in a small population. Unlike natural selection, genetic drift is random
The frequency of a certain allele changes. In this instance, there are two kinds of colors for the frogs, the green frogs (G) and the brown frogs (g).
There are two ways genetic drift can occur, the Bottleneck Effect or the Founder Effect. In this situation our frogs were affected by the bottleneck effect. A natural disaster or some other random event drastically reduced the populations numbers as well as it's genetic variation.
The same thing would happen with the founder effect. In that case, a few frogs from the original population would have to start a new colony elsewhere and in doing so, reduce the genetic variation that came with the smaller population.
In this next generation, the brown trait (g) was completely lost with the lose of genetic variation in the population of the frogs.
Since genetic drift causes large changes in populations over short periods of times, it can lead to evolutionary changes. Though it's important to note that despite this, these changes do not produce adaptations because they are completely random.