Sympatric speciation occurred in the cichlid fishes within the same lake in Africa. How could this occur without physical separation?

There are differences in the water in different parts of the lake. Some areas receive a lot of sunlight while other deeper areas are darker. The species have different retinal pigments that are more sensitive to blue light in the shallow, lighter areas and other species have retinal pigments that are more sensitive to red light in the deeper areas. The males in the shallow part of the lake are colored blue so the females of that species choose blue males for mates while the males are redder in color deeper in the lake. Thus, mutations in genes that affect color perception are likely to affect a female's choice of mates. Such mutations are probably the way sympatric speciation occurs in these fish.