The allele frequency for the green skin color before the climate change occurred would be 0.2, and after the climate change, it would be 0.81.
The reason why the brown lizard, which initially had greater relative fitness than the green lizard, reduced in number drastically is because the environmental conditions changed due to increased rainfall and growth of shrubs and small trees, which favored the green lizard population. This allowed the green lizard population to acquire new adaptations that helped them thrive in the changed habitat and increase in number, resulting in a decrease in the brown lizard population.