Some varieties are more sensitive to blue than red, others are more sensitive to red. If such variation in susceptibility also exists within a species, females with maximum sensitivity to blue will see the blue males most clearly and are therefore more likely to mate with them, while red males are most prominent for females with maximum sensitivity to red.
We assume that small changes in the eye can change the sensitivity to the different colors, and thus the sexual preference, and indeed found variation in the preference of females. Then the sexual selection by females can increase the differences between color variants and further divide the groups so that they can develop into species. This also explains why haplochromine cichlids have developed many species especially in lakes with clear water. In turbid water, no colors are visible, so sexual selection cannot increase differences between color variants and speciation cannot take place in this way.
About the Females
Although the females are usually the picky and the males are the brightly colored genus, we also discovered species where it is the other way around. In this case, too, color polymorphisms exist, but now in the females, and again there are strong indications of sympatric speciation (the difference in behavior is the barrier that causes the groups to diverge), but now due to male selectivity. Sexual selection is expected to lead to faster speciation than natural selection because it influences the preferred trait (the bright color) and directly increases the degree of isolation between two divergent groups.
As new species emerge, it is not certain that they will continue to exist side by side. The two young species are said to be in the same area and very similar, for example in food choice. In Lake Victoria, the amount of food available for African Cichlids fluctuates and there is evidence of competition for food. If there are hardly any differences between two species, one of the species will prevail and the other will disappear. Therefore, there will be strong selection to increase the differences in the eating behavior of the species. Species created by sexual selection can only survive if they grow apart quickly enough in terms of food and other resource use.
The extensive specializations of the African cichlids suggest that these fish can quickly change their menu and eating habits. Another indication for this is that subtle differences in the use of food sources are often seen between differently colored sister species. In one documented example, the red species eats mainly plankton above rocks, while the blue mainly eats insect larvae among the rocks.
What is crucial for those quick menu changes; what makes hundreds of variations on a theme possible? When looking at other animal families, it is striking that families with many species, as far as that has been investigated, often have a construction plan with more loose, that is to say more or less independently moving parts than other families. A large number of independently moving parts offers more possibilities for variation in evolution and allows a greater diversity of species.