Science Literature

Cichlid fish have been called "a crucible for evolution studies". Lake Victoria, in Eastern Africa, contains over 500 species of cichlid fish - all members of the same distinctive family, but differing in numerous details of morphology, behaviour and genetics. Since these species can reasonably be inferred to be linked by common ancestry, they are ideal targets for speciation research. Since many of these fish occupy similar ecological niches within the lake, sympatric speciation mechanisms (not involving geographical isolation) have been considered highly probable.

Speciation - something to shout about? (source here)

The new research focuses on light: colouration and factors affecting vision.

"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."

The key finding has been an association between ambient light, the colour of the fish and the genetics of colour vision.

"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."

This association, although not surprising, is significant for understanding the ecology of the cichlids. Jenny Boughman, an evolutionary biologist, is quoted as saying: "They show a clear segregation between what the fish see, their colour and where they live." The inference follows that visual sensing plays a significant part in male selection. The authors have not argued a convincing case here:

"But [Boughman] 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."

Nevertheless, a start has been made: "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 [. . .]." But these preferences on the part of some animals falls short of rigorous documentation and the species mechanism is still at the level of hypothesis.

So, what has been achieved? Is this a textbook example of evolution? All the reports of this research emphasise the observations that have been made, and it is correct to say that these observations confirm an association of vision-related parameters. Crucial observations of mating behaviour are in short supply and there are no direct observations of speciation. Yet the Nature summary claims the research provides "clear evidence that speciation can occur through sensory drive without geographical isolation". Nature News is more realistic in its assessment: "The new work is the strongest evidence yet that differences in sensory input - in this case, vision - can give rise to new species."

"The strongest evidence yet" involves a correlation between the visual system, body colour and ecology. Instead of this being used to support a hypothesis of sexual selection based on body colouration, the authors claim to have demonstrated sexual selection in action. This has been picked up by the media as fact: "a fish species in the cichlid family has been observed by scientists in the act of splitting into two distinct species in Lake Victoria" (Source). The cover of Nature proclaims that this is "a textbook example of evolution in action".

Let us suppose that the hypothesis is tested and confirmed, and the "sensory drive speciation" is validated. What are the implications for our understanding of evolution? It means that an ancestral fish population can split into two or more populations on the basis of colour. The daughter populations have differences in sensitivity to light frequencies and differences in body colouration. These may be accompanied by other ecological adaptations. There is no new genetic information - just fine-tuning of existing genetic systems. There is no evidence that these new species lack the potential to interbreed. Indeed, the differences are so slight that hybridisation to produce fertile offspring can be predicted with some confidence.

The claim of "evolution in action" will mean different things to different people. To ID scientists, it means that relatively trivial variations can and do occur in living things. These variations allow organisms to diversify and prosper in new ecological niches as they become accessible. However, these variations have nothing to do with the origin of cichlids, eyes or complex specified information. To most evolutionary biologists, apparently, it means that all the big questions of biological origins can, in principle, be solved. If we can demonstrate speciation in action, then we can rest assured that we also have an explanation of the origins of families, orders, phyla, organs and organelles. Evolutionary biologists can then repeat the mantra that nothing in biology makes sense without evolution.

The punchline: ID scientists are not opposed to the teaching of evolution in schools, but want it taught properly - allowing critical appraisal and the recognition of spin. Let speciation in cichlid fish enter the textbooks, not as a proof of evolution, but as an example of how evidence is brought to bear on current hypotheses of the origin of species.

Speciation through sensory drive in cichlid fish
Ole Seehausen, Yohey Terai, Isabel S. Magalhaes, Karen L. Carleton, Hillary D. J. Mrosso, Ryutaro Miyagi, Inke van der Sluijs, Maria V. Schneider, Martine E. Maan, Hidenori Tachida, Hiroo Imai & Norihiro Okada
Nature 455, 620-626 (2 October 2008) | doi:10.1038/nature07285

Abstract: Theoretically, divergent selection on sensory systems can cause speciation through sensory drive. However, empirical evidence is rare and incomplete. Here we demonstrate sensory drive speciation within island populations of cichlid fish. We identify the ecological and molecular basis of divergent evolution in the cichlid visual system, demonstrate associated divergence in male colouration and female preferences, and show subsequent differentiation at neutral loci, indicating reproductive isolation. Evidence is replicated in several pairs of sympatric populations and species. Variation in the slope of the environmental gradients explains variation in the progress towards speciation: speciation occurs on all but the steepest gradients. This is the most complete demonstration so far of speciation through sensory drive without geographical isolation. Our results also provide a mechanistic explanation for the collapse of cichlid fish species diversity during the anthropogenic eutrophication of Lake Victoria.

See also:

Kirkpatrick, M. & Price, T. Sensory ecology: In sight of speciation, Nature 455, 601-602 (2 October 2008) | doi:10.1038/455601a

Gilbert, N. What you see is how you evolve, Nature News, 1 October 2008 | doi:10.1038/news.2008.1144