A male Pundamilia pundamilia from a clear water site where red and blue Pundamilia are genetically differentiated species.Inke van der Sluijs

What fish see — at least in one African lake — could be the driving force that causes them to evolve into new species.

Evolutionary science typically holds that new species are born when populations become isolated from one another, forcing them to adapt differently. The new work is the strongest evidence yet that differences in sensory input — in this case, vision — can give rise to new species.

Biologists led by Ole Seehausen, from the University of Bern in Switzerland, studied cichlid fish in Lake Victoria in east Africa¹. The lake contains more than 500 species of cichlids, whose extraordinary diversity provides a crucible for evolution studies. But, says Seehausen, "the mechanism for how you could have that many species in one lake is not clear".

Red fish, blue fish

His group thought that vision might be part of the reason. Water clarity and ambient light vary considerably throughout the lake, and fish of many species range in colour. In shallow parts, blue light is dominant, and the waters are populated by blue males of the species Pundamilia pundamilia. As the water deepens, red light becomes increasingly dominant, and greater numbers of red-coloured males of the species Pundamilia nyererei are seen.

A male Pundamilia from a turbid water site where only a single intermediately-coloured Pundamilia species exists.Ole Seehausen

Previous studies have shown that natural selection acts on vision to tune eyes to their environment so that individuals can see best what they eat, what eats them and which other members of their own species are around. And the researchers had earlier identified variants in one of the genes responsible for tuning the fish's vision to different colours². In the new study, they found that some of the variants are more sensitive towards red light; fish with these gene variants lived deeper in the lake. The other variants are more sensitive to blue light; fish with these variants lived in shallower water.

"We found a strong association between the visual gene and fish colour," says Seehausen. "Red fish have the gene that shifts vision towards picking up red light, and the blue fish have visual genes that pick up blue light."

Boy fish, girl fish

The group also found that some females with blue-biased vision prefer to mate with blue males, and red-biased females mate with red males — suggesting how the vision differences could lead to changes in mate choice and, ultimately, to the birth of a new species.

"It is a really nice paper and has an impressive use of data," says Jenny Boughman, an evolutionary biologist at the University of Wisconsin, Madison. "They show a clear segregation between what the fish see, their colour and where they live."

But she says more work is needed, such as evidence that shows it is the difference in female vision that causes them to pick the differently coloured males. Only that, she argues, would prove that vision is driving the speciation.

• References

  1. Seehausen, O. et al. Nature 455, 620–626 (2008).
  2. Carleton, K. L. , Parry, J. W. , Bowmaker, J. K. , Hunt, D. M. & Seehausen, O. Mol. Ecol. 14, 4341–4353 (2005).