Improbable Destinies by Jonathan B. Losos

Author:Jonathan B. Losos
Language: eng
Format: mobi, epub
Publisher: Penguin Publishing Group
Published: 2017-08-08T00:00:00+00:00

The rolling hills and prairies of Nebraska are known for their rich, fertile land—brown, earthy, packed full of plant goodness. There’s a reason the name of the state university’s football team is the Cornhuskers. But not all of the state’s land is so productive. About a quarter of the state is in the Sandhills region, an area in which the soil is sandy and light-colored, composed of quartz specks blown east from the Rocky Mountains about eight thousand years ago. Crops grow poorly there and most of the area has never been cultivated.

That’s not to say that nothing flourishes on the Sandhills. Quite the contrary, the area is biologically rich, so distinctive that it’s recognized as its own ecoregion by the World Wildlife Fund. The soil affects the region’s biota not only by dint of its low fertility, but also as a result of its light coloration. All around the world, small animals have evolved to match their background, all the better to avoid being detected by their predators. On old lava flows, lizards, mice, grasshoppers, and other animals have evolved to be much darker than they are elsewhere. Conversely, on light-colored soil, animals evolve a pale complexion to blend in with the sandy substrate. The Nebraska Sandhills are no different—populations of many species there are lighter than members of the same species on nearby darker soil.

This phenomenon has long interested me, ever since my college days when I read John Endler’s book on speciation. Endler cited background-color matching as some of the earliest and strongest evidence that natural selection could overpower the homogenizing effect of genetic exchange between populations. The border between black lava rock and gleaming white sand is quite distinct—in places you can stand with one foot on each. Mice, lizards, and grasshoppers easily move back and forth from one surface to the other.

Yet, despite their proximity, populations on the different surfaces are often very different in coloration, appropriately matched to where they live. Members of the two populations can meet near the border, but any offspring from such encounters are scrutinized by natural selection and genes for the mismatching color quickly weeded out. The fact that some of these environments—the Nebraska Sandhills, some lava flows—only appeared recently indicates that color adaptation has occurred rapidly, providing further evidence for the power of natural selection even in the presence of gene flow. These are the animal analogs to plants living on or off old mine sites or on the different Rothamsted plots.

Research on deer mice—so named for their running and jumping agility—has been particularly influential. Naturalists in the middle of the last century noted many cases in which neighboring populations occurred on different-colored substrates and had correspondingly different fur coloration. The presumed explanation was camouflage—rodents are eaten by many visually oriented predators, so natural selection would drive populations to become similar in coloration to the background on which they occurred.

A University of Michigan biologist, Lee Dice, even tested the idea in the lab. Dice took a normal-sized room, covered it with soil, and released different-colored mice.

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