in the tropics, two lakes are connected by a short stretch of a river that flows from lake a to lake b. because the current in this river is not very strong, a single fish species has easily passed from one lake to another in both directions for millions of years. then, hundreds of thousands of years ago, a rockfall prevented the fish species from traveling between the lakes. this caused the single species to evolve into two species. eventually, some of the fish in lake a evolved mutations that allowed them to eat mollusks, which the original species in lake a couldn't. as a result, these two groups became genetically distinct and unable to interbreed. what type(s) of speciation has (have) influenced the evolution of the species in lake a over time?