1. Natural selection.
This environmental change, by resulting in two different types of lizards that match the colour of the environment in which they are in, indicates that there was a natural selection, probably from the lizards' predator, that could now see them better if they were in the vegetation not corresponding to their colour leading to the remaining of dark lizards in a dark vegetation, and light lizards in a lighter vegetation.
2. Allopatric speciation
This type of speciation is also known as geographic speciation. It is so because it occurs when populations of the same species are isolated from each other in such a way that there are no genetic interchanges and are pushed to evolve into different species. In this case, the lizards were isolated by the stream that divided the two new areas.