Color vision gives birth to new species

Sensitivity of the female visual receptors crucial for mate choice

Cichlid of the species Pundamilia nyererei © Eawag
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In many species, the colored wedding dress of the male decides whether it is preferred by females or left lying. A study on colorful fish now shows that this has less to do with beauty ideals than simply with the sensitivity of the female eyes, which varies by adapting to their environment. These partner preferences can be so strong that new species form - without spatial separation of populations.

When asked what role selection plays in the formation of new species, evolutionary biology still has few answers. The development of very colorful perch in African lakes, which took place in a few thousand years - a short time in evolution - supports the thesis that sexual preferences, ie preferences in the choice of sexual partners, can contribute to speciation, even without populations spatially be isolated from each other. The assumption suggests that the selection in the case of cichlids is due to a different perception of colors. Now, a study just published in the journal Nature provides evidence for this assumption for the first time.

Receptors in the female eye crucial

The evolutionary biologist Ole Seehausen of the Swiss Water Research Institute Eawag and the University of Bern and his co-authors show that cichlid fish from Lake Victoria, whose eyes are better adjusted to blue, are more likely to seek blue-colored males. Females whose receptors recognize the red color spectrum better, choose males in a red wedding dress. The different pigments in the receptors in the eyes were able to differentiate the team based on their DNA and protein sequences.

The DNA sequence of the genes that underlie the eye pigments also shows that the specialization is not a coincidence, but in turn originated under natural selection. Depending on the depth of the water in which the fish are, the sensitivity in color vision varies. Females in deeper water see red better, those in shallow water blue. This adaptation of their receptors to the prevailing light color in the environment gives the fish an advantage in a certain depth range. They can orient themselves better there and find, for example, more food than a non-adapted conspecific. At the same time, the males have evidently adapted to this situation: in the deeper water, males that develop red wedding colors to sexual maturity dominate those in shallow water with blue colors.

New species emerge when the light spectrum changes slowly with water depth. So there is enough room in each light area that the different genetic variants of the fish can also exploit the color advantage in their niches. In the case of Lake Victoria, such zones are on shallow to medium banks with relatively clear water. display

Explanation for drastic drop in species

The new results not only show a path of species development, they also provide a plausible explanation for the dramatic decline in biodiversity that has taken place in Lake Victoria over the past 25 years. The overburdening of the lake caused by agriculture, deforestation and surrounding large cities has greatly promoted the turbidity in the lake. This leads to the fact that the light conditions change drastically after just a few meters of water.

The different Ncological niches are therefore so close today that the mechanism of genetic adaptation to them can no longer play. Thus, the authors found that in places with dull water instead of a red and a blue species, there can only be an intermediate form, which is not specially adapted to any of the light niches. This mix of species, driven by environmental change, has most probably contributed greatly to the fact that within just a few generations of more than 500 cichlids in Lake Victoria, only about 250 species still exist today.

(Swiss Water Research Institute Eawag, 06.10.2008 - NPO)