在线英语听力室_免费在线英语听力学习网站

Hundreds of millions of years ago a humble¹ fish swam in the lakes and rivers of the supercontinent Gondwana. Eventually Gondwana broke apart, becoming the continents we know today. And the descendants of that fish, now called cichlids, continue to swim the fresh waters of both Africa and South America. 

Cichlids have some of the most incredible visual systems known. Humans have genes² that code for three different types of visual pigments³, called opsins. Cichlids have seven.

"But what is interesting within cichlids, which is this group of very diverse fishes, is that they can express different sets of these seven genes. So they only express three typically, but different species express different groups of these seven genes."

Biologist Daniel Escobar-Camacho, from the University of Maryland in College Park.

"So for example, we have opsin genes that code for the blue, green, and red opsin…whereas cichlids have genes that are sensitive to UV, violet, blue, blue-green, green, light green, and red." 

But selection pressure has kept only some of those genes intact.

African cichlids, whose visual systems are well studied, evolved in fairly clear, calm, blue lakes with plenty of sunlight. And it was known that they’ve maintained the genes for seeing short wavelength⁴ light, at the blue end of the spectrum⁵. But their South American counterparts live in the murky⁶ waters of the Amazon River basin, bathed mostly in reds and oranges. 

Escobar-Camacho analyzed⁷ DNA⁸ from three different Amazonian cichlids: the freshwater Angelfish, the Discus, and the Oscar, all of which are also popular in home aquariums⁹. He discovered that each species has completely lost at least one of the seven opsin genes, and some have even lost two. But they've each lost different genes. 

“What is interesting is that they are expressing genes in the retina that allow them to be long wavelength-sensitive. And this is in concordance with the light environment in Amazon waters, because Amazon waters transmit long wavelengths¹⁰ best.”

In other words, Amazonian cichlids were most sensitive to red and orange light, which makes sense because Amazonian rivers filter out most of the blues¹¹ and greens. The results are in the journal Molecular¹² Ecology. [Daniel Escobar-Camacho, Erica Ramos, Cesar Martins, & Karen L. Carleton. The opsin genes of Amazonian cichlids.]

The finding is consistent with an idea called the "sensitivity hypothesis," which holds that a color visual system evolves by adapting to the dominant¹³ wavelengths of light in the environment. In other words, what you get is what you see.  

—Jason G. Goldman