You have got to read this article from Nature magazine. Many of you may not know Nature magazine, although I suspect some of you do. It is a fantastic magazine that covers the broad spectrum of the sciences. I’ve come across it, as I’m sure most of you have, based on the fact most newspapers of general circulation around the country quote either Science magazine or Nature magazine, or one of their derivative publications, for notice of publication of the most recent advancements in science.
This particular article reflects the finding that the adaptation of a fish’s eye to its visual environment biases the females to mate with males according to their specific coloration. Even more substantially, it also suggests that it can contribute to the formation of new species.
Apparently, according to the article, since females with blue biased vision tend to mate only with blue males and females with red biased vision mate only with red males, what you see is what you get.
Looked at differently, beauty really is in the eyes of the beholder! Enjoy the article.
In sight of speciation
Mark Kirkpatrick and Trevor Price
Adaptation of a fish’s eyes to its visual environment can bias females to mate with different males according to their coloration. This sensory preference can contribute to the formation of new species.
How and why do barriers that prevent mating between species evolve? On page 620 of this issue, Seehausen et al.1 present a rich and eclectic data set that suggests a key role for vision in African cichlid fishes. It has been shown in other fish that natural selection tunes eyes to their visual environment, so that individuals can best see not only what they eat and what eats them, but also members of their own species2–4. Seehausen et al. carry this story a step further with work on fish in which the males are either red or blue (Fig. 1), and which have genetic variation for visual sensitivity to those colours.
In some populations, females with bluebiased vision seem to mate only with blue males, whereas red-biased females mate only with red males. The inference is that natural selection acting on the visual system contributes to reproductive barriers and the formation of new species. In short, what you see determines what you get, and with whom you get it on. More controversially, the authors suggest that these barriers might arise within a population, and do not, as has previously been thought, require a phase in which red and blue populations evolve in geographical isolation.
The biological and ecological setting for this story is dramatic — the cichlid fish in the Great Lakes of Africa. These fish are the most rapidly speciating organisms on Earth, and this explosion of life has produced a panoply of colour, morphology and behaviour, a sampling of which can be seen at your local pet shop. The fish in Lake Victoria, where the present study1 was done, show a fantastically high rate of speciation. More than 500 species inhabit the lake. They may have originated just a few hundred thousand years ago5, and possibly went through a period of large-scale interbreeding 20,000 years ago6.
The lake has diverse visual environments. Starting at the shore and descending along the lake bottom, red becomes increasingly dominant in the ambient visual spectrum. This spectral shift is rapid at some sites and more gradual at others. To study how the fish have adapted to these conditions, Seehausen et al. wanted to know what the fish see. They identified genetic variants (alleles) in one of the opsin genes responsible for tuning the fish’s visual sensitivity to different colours. By expressing these genes in vitro and measuring the absorption properties of the resulting proteins, they found some variants that are redbiased in their sensitivity and others that are blue-biased. The red-biased variant is typically found in fish living at greater depths than the blue-biased one.
The numbers of males with red, blue and intermediate coloration vary between populations. At sites where the spectral shift is neither very rapid nor very gradual, notably Makobe island, blue males are confined to the shallows and red males to greater depths. At this site, the great majority of blue males carry the blue-biased opsin variant, whereas most red males carry the red-biased one. The two colour morphs also show differences in other genetic markers, suggesting that they are nascent species. At sites where the spectral shift is rapid, however, the colour forms interbreed, presumably because they encounter each other frequently.
Is beauty just in the eye of the beholder? In mate-choice experiments using fish from controlled crosses, Seehausen et al. find that the opsin variant alone does not strongly determine mating preference. Segregation of the colour morphs by depth in the lake must mean that the fish mainly encounter and hence mate with their own kind. It is not difficult to imagine that fish prefer to spend time in habitats in which they see best — that is, visual tuning could generate a type of habitat preference that contributes to speciation above and beyond its effects on mate choice.
But is even this enough? Other findings point to an additional mechanism that complements reproductive isolation via vision. The females of these remarkable fish brood their eggs in their mouths, then guard the young fry after they hatch. In experiments reported last year, Verzijden and ten Cate7 swapped eggs between the mouths of red morph and blue morph mothers. Females raised from the experimental broods strongly preferred males from their foster morph over those of their own morph (Fig. 2). As females of the two species look very similar, it is unclear whether the offspring preference is based on colour or some other correlated cue such as odour. Regardless of that, learning at a young age (sexual imprinting) apparently contributes to reproductive isolation in these cichlids, as it does in other groups such as birds8. The implication is that assortative mating — the tendency of like to mate with like — can arise whenever male characteristics diverge in response to differences in the environment, which might happen even without divergence in the opsin pigments. It remains to be seen if imprinting, vision and perhaps other mechanisms have been sufficient to generate new species without geographical isolation.
An intriguing observation mentioned by Seehausen et al.1 is that the red- and blue-biased opsin alleles are evolutionarily much older than the species studied here. Red and blue colour morphs are found in other species of cichlid9, suggesting that the colour polymorphism may also be ancient. Perhaps one key to the spectacular species radiation of African cichlids is that they inherited from distant ancestors a trove of genetic variation for sensory systems and male signals, possibly contributed during the inferred episode of interbreeding 20,000 years ago. This variation is entrained again and again in speciation events. To systematists, these events represent independent nodes on the evolutionary tree. From the fish’s point of view, however, they are perhaps more like an evolutionary play that is re-enacted, night after night, with the same genetic cast.
Mark Kirkpatrick is in the Section of Integrative
Biology, University of Texas, Austin, Texas 78712,
USA. Trevor Price is in the Department of Ecology
and Evolution, University of Chicago, Chicago,
Illinois 60637, USA.
1. Seehausen, O. et al. Nature 455, 620–626 (2008).
2. Boughman, J. W. Nature 411, 944–948 (2001).
3. Maan, M. E., Hofker, K. D., van Alphen, J. J. M. & Seehausen, O. Am. Nat. 167, 947–954 (2006).
4. Cummings, M. E. Evolution 61, 530–545 (2007).
5. Genner, M. J. et al. Mol. Biol. Evol. 24, 1269–1282 (2007).
6. Seehausen, O. et al. Proc. R. Soc. Lond. B 270, 129–137 (2003).
7. Verzijden, M. N. & ten Cate, C. Biol. Lett. 3, 134–136 (2007).
8. ten Cate, C. & Vos, D. R. Adv. Study Behav. 28, 1–31 (1999).
9. Seehausen, O. & Schluter, D. Proc. R. Soc. Lond. B 271, 1345–1353 (2004).
NATURE|Vol 455|2 October 2008
Original writing date: October 13, 2008
Article writing date: October 2008