Sex, speciation, and fishy physics
Holding a prism up to a bright window demonstrates a basic principle of optics that the plain white light radiating from our sun is actually made up of a rainbow of colors. Through air, these colors of light all travel in about the same way, but in water, different colors travel differently. Water molecules tend to absorb reddish light, leaving the blue light to travel towards the depths of large bodies of water. Because of this, deep ocean waters look blue. However, everything changes when the water is clouded by particles. Just picture a silt-clogged river or lake. Such sediment particles are particularly good at absorbing bluish light the opposite of water molecules. So when the sun shines on cloudy waters, blue light is present near the surface, but just a few meters down, most of the blue light will have been absorbed and mainly reddish light will penetrate.
Where's the evolution?
To understand how the physics of light can promote speciation, picture a lake with slightly cloudy water. Near the surface, blue light dominates the visual environment, but in deeper waters, red light does. A fish population lives along the lake's shore where it slopes from very shallow water to deeper water so some of the fish spend more of their time in blue light and some spend more of their time in red light. Like all populations, the fish have genetic variation that is, individual fish have different gene versions from one another and some of this variation affects the fishes' ability to see different colors. Some fish have genes that enable them to see blue light better, while other fish have a red light advantage. Because of the differential penetration of light into the lake, fish with gene versions sensitizing them to blue light have an advantage in shallower waters because they can better find food and spot predators there, while fish tuned to red light have an advantage in deeper waters. So in different parts of the fishes' habitat, different color-sensitivity genes are favored by natural selection. Over many generations, if the fish don't move too much within their range, blue sensitivity will evolve to be more common among fish living near the surface and red sensitivity will become more common among fish living further down the slope.
By itself, natural selection acting on light sensitivity can cause something of a rift in the population, but when sexual selection is considered as well, the divergence is amplified. To see how, add the male fish to the equation. They have some variation in color. Some males have genes for blue coloration, some have genes for red coloration. This matters because female fish are choosy about their mates and tend to pick brightly colored males to father their offspring. In this scenario, blue males living in deep waters would have trouble finding mates for two reasons: (1) there is little blue light around, so they look duller than red males, and (2) the females living in deep waters tend to be less sensitive to blue light than they are to red. On the other hand, red males living in deep water would be winners on both counts: their coloration makes the most of the available red light, and the females living at those depths tend to carry genes that make them extra-sensitive to red light. Bottom-dwelling blue fish face a long series of lonely nights, while bottom-dwelling red fish get all the girls. And of course, the opposite is true near the surface. Over many generations of sexual selection acting in this way, the two parts of the population may diverge. Though they live right next door to one another, the fish will evolve to prefer to mate with other fish that share their coloration, light-sensitivity, and habitat. Over time, the two sub-populations may even cease to mate with one another entirely and evolve enough differences to be considered separate species.
Although the scenario described above is a hypothetical one based on evolutionary theory, biologists have now discovered strong evidence that this process has actually occurred in the cichlid fish of Lake Victoria. They've observed, along with other lines of evidence, many neighboring species pairs in which the surface-dwellers tend to be blue and blue-light-sensitive, while the deeper fish tend to be red and red-light-sensitive. Biologists are particularly excited about this discovery because it may represent an unusual mode of speciation. The sort of speciation that is easiest to gather evidence about is allopatric speciation speciation that occurs partly through geographic isolation of the emerging species. In contrast, these cichlid species may have evolved without any physical separation at all.
Unfortunately, the cichlids of Lake Victoria face an uncertain future. In the past few decades, many cichlid species have disappeared. Some of these may have been pushed to extinction by invasive species, but others seem to have simply collapsed into one another. Deforestation, agricultural run-off, and pollution have caused the lake's water to become much murkier. This has made it more difficult for the fish to choose mates based on color and has caused some species that rely on visual signals for mate choices (like the fish described above) to interbreed resulting in fewer distinct species of fish and fish with duller colors. The recent research into how optics has influenced cichlid evolution shines a light on the processes that contribute to Earth's fabulous biodiversity, but also highlights how easily that diversity can be snuffed out.
Discussion and extension questions
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