Answer:
<em>This phenomenon occurred due to natural selection and evolution.</em>
Explanation:
Natural selection tends to favour organisms carrying the alleles which provide better traits to live in an environment. Evolution tends to change the allele frequency of a species with time depending on the adaptations which are more perfect to survive in an environment.
In the past, blue frogs lived in lakes. The colour of the lake matched with the colour of the frogs. As a result, these frogs could escape from their predators and hence were better adapted to survive in such an environment.
When the lake dried, the blue frogs were not adapted to live in the muddy environment as their colour no longer helped them to escape from the predators. As a result, natural selection favoured those organisms carrying brown colour and evolution changed the allele frequencies and increased the number of brown frogs.
