Act on the cns to suppress or slow bodily processes and reduce overall responsiveness.

Explanation:

The allele are fixed because there is no variation that exist in the population and the allele are almost completely the same hence no new combination of gene the same Allele will continue to reoccur

Variability exist because of random mating that occurs in the population initially when the toad filled up the entire central valley

Random mating ensures fitness in the initial population

Genetic drift acted upon the population drifting away allele that are very few in population or that occur less in the population and perpetuating alleles that frequently occur leading to a fixed allele.

There was a decrease in the population as a result of the change in the environment.