The cause of speciation in this example is Geographic isolation.
Speciation is the process by which a new species forms from an existing species. In this example, a shift in climate causes the river to permanently dry up, severing the connection between the two lakes, and the fish species found in these two lakes are isolated from each other. This geographic isolation means that the two populations of fish can no longer interbreed and exchange genetic information.
Over time, these two isolated populations may evolve in different ways as they adapt to the different conditions in their respective lakes. If these changes are significant enough, the two populations may eventually become so different that they can no longer interbreed. At this point, they are considered to be separate species.
Geographic isolation is a common cause of speciation, as it separates populations and allows them to evolve in different directions. The other options are not the cause of speciation in this example. The bottleneck effect is when a population goes through a drastic reduction in size and the founder effect is when a new population is established by a small number of individuals from a larger population. Random genetic drift is a change in the frequency of alleles in a population due to chance events.