DNA analysis had already indicated that the 500 or so cichlid species now living there share a common ancestor. Whatever it is that causes speciation, cichlid fish must have it in spades. Exactly what constitutes a species is hotly debated. But for practical purposes, if two distinct populations exist that cannot or will not breed with one another, they can be considered separate species. The novelty comes with random genetic mutation.
Most mutations are harmful, some have no effect on survival, and very few lead to new species.
Karel Liem, from Harvard University, has suggested that cichlids underwent their own unique key change. By mechanically uncoupling their upper and lower pharyngeal jaw, the fish have hugely increased their ability to adapt to specialist diets. As a result, adaptive radiation is common. The cichlid's isolationist lifestyle helps to keep these evolving groups separate from their close relatives. George Turner from the University of Southampton and his colleagues used DNA analysis to measure the relatedness of cichlid populations living near adjacent rocky headlands in Lake Malawi.