The poster child for evolution may have finally revealed its secret. More than 1000 closely related but different species of cichlid fish live in Lake Malawi in south-east Africa – more than in any other lake in the world.

“They are remarkable,” says Christopher Scholz at the University of Syracuse in New York. The huge number of closely related species living together has meant they feature prominently in models of species diversification. But what made them so diverse has remained a mystery.

Some think environmental forces drove the diversification, others that the underlying cause was biological, says Scholz. For example, some females are colour-blind to males that are a different colour to them, which can drive sexual isolation between different groups of fish.


To try and settle the debate, Scholz and his team examined sediment records from the lake covering 1.3 million years. They found that over this period the water levels dropped by more than 200 metres around 24 times.

Forced diversification

These dramatic changes would have changed the habitat, says Scholz, as less water in the lake would shift the rocky shoreline inwards, alter the pH and salt levels of the water, and even separate the lake into smaller ones. This would ultimately force the fish to adapt to the new conditions and diversify.

What is remarkable is that even after the lake reformed and the fish came back together again, they managed to stay distinct, says George Turner at Bangor University in the UK.

Understanding how they achieved this will require studying the genetic basis for their subtle differences in behaviour, mating signals and feeding apparatus, he says.

Thomas Kocher at the University of Maryland isn’t convinced that the changes in the lake water levels alone drove the diversification of cichlids.

What the findings do show, however, is why the cichlids came to dominate the lake: they were able to “out-evolve” other groups of fish while these changes were occurring, he says.

But cichlids aren’t the only group of animals with an enormous number of species, says Scholz. For example, there is a wide variety of snails in Lake Tanganyika, just north of Lake Malawi. He now wants to see whether similar changes may explain biodiversity in other lakes as well.

Journal reference: PNAS, DOI: 10.1073/pnas.1512864112

Read more: “Dark secret of the lake”

Image credit: Cristian Umili / VWPics/age fotostock/ Superstock