The many cichlid species in each African lake evolved in the same body of water, so much of their speciation is thought to be sympatric. The main mechanism is sexual selection leading to behavioural isolation.In many cichlid species, females choose their mates based on the colour of the males. The best-studied example is from Lake Victoria:1. Pundamilia pundamilia males are blue and live in shallow water, where blue light is common.2. Pundamilia nyererei males are red and live in deeper water, where the light is mostly red and orange.The visual pigments in the females' eyes differ between the two species, so each female is more sensitive to, and prefers, males of the colour best seen at her depth.The process of speciation is:1. Within one population, some females prefer red males and some prefer blue males.2. Females mate only with males of their preferred colour, so alleles for colour and for preference are passed on together.3. Gene flow between the two colour groups falls. Each group becomes reproductively isolated by behaviour, even though they live in the same lake.4. Different selection pressures at different depths, and different mutations, make the gene pools diverge further.Evidence for this comes from experiments: when females are given a choice of males under white light, they choose males of their own species. Under monochromatic (orange) light, when colours cannot be distinguished, females mate with males of both species and the hybrids are fertile. This shows that colour-based mate choice is what keeps the species apart.