Researchers were able to trace for the first time an evolutionary change in an animal, which is the result of changing natural environmental conditions
Carol Keisuk Yoon
A pocket mouse with dark fur, and a mouse of the same species with light fur. The color change makes it difficult for predators. Photographs: New York Times
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The gerbils, living in the deserts of the southwest of the United States, have fur that is the color of the sand - so they are well camouflaged when they rush across the light brown rocks. But there are also pocket mice whose fur color is different: in desert areas where ancient lava flows that solidified have left strips of blackened stone, you can see mice with dark fur, providing them with an equally successful camouflage.
In an article recently published in the journal "National Academy of Sciences" "Proceeding of the", researchers reported that they had identified the gene responsible for the evolution of the dark fur color in these mice. According to scientists, this is the first documentation of the genetic changes underlying adaptive change due to evolutionary pressures originating from natural factors. In other known cases, what drove the evolutionary changes was the influence of humans, and there were researchers who claimed that these changes go beyond natural evolutionary change, since they usually involve a large and direct pressure that causes the destruction of the majority of the population.
Well-known examples of changes as a result of human influence are the resistance to pesticides developed by insects, and the increase in the number of spotted moths with dark wings in the USA and England due to air pollution with soot particles.
The researchers - Dr. Michael Nachman, Dr. Hopi Hoekstra and Susan D'Agostino from the University of Arizona - tested mice living on a layer of lava rock in Arizona, and on light-colored rocks nearby. On closer inspection you could see that the bright gerbils had the same pattern of fur color as normal lab mice with sand colored fur. In this pattern, known as "agoti", the hair is black near the root, yellow in the middle and black at the tip. The dark gerbils had completely black hairs.
Initially, the researchers tested two genes called "agoti" and .Mc1r. When they examined the DNA sequences in light and dark mice, it seemed that changes in "agoti" were not related to the differences between light and dark fur. However, the researchers found that a specific cluster of mutations in the Mc1r gene was found in every dark-furred mouse. "We now have all the pieces of the puzzle in our hands, in a natural environment," said Nachman.
Nachman pointed out that although the new study points to the Mc1r gene as the key gene for darkening the fur of the mice living in the lava layer in Arizona, the team discovered that the same mutations were not found in dark mice living on another lava layer in New Mexico. "The same dark color evolved independently in two different populations," said Nachman, "using different genetic solutions to the same evolutionary problem."
One would think that for nocturnal creatures like gerbils, fur color is meaningless. But in experiments conducted at the beginning of the 20th century, researchers released light and dark mice on light and dark backgrounds, and discovered that owls - which are one of the main predators of mice - are able to easily locate in the dark a mouse that is on a background that does not match its fur.
Many species of animals are divided into light and dark forms, including the wood hamster, the chipmunk and the striped squirrel. There are even ladybird beetles that are completely black. "Many of the dark shapes are related to a certain type of rock layer that these animals live on, or to the frequency of fires in forested areas," said Mike Majeros, an evolutionary geneticist at the University of Cambridge.
But while dark forms of many animals are common, the spotted moth is disappearing, partly due to reduced air pollution. "We have about 15 or 16 more years," Majros said, "before these black shapes disappear completely."
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