Natural selection, or “survival of the fittest”, is an evolutionary shift in a species. It occurs when member or members of a species possess specific genetic traits (natural or a mutation) that are beneficial to their survival in a particular environment. Those that survive because of that trait are obviously the only left to reproduce and the trait carries over to their offspring, and becomes common to that species. This theory was observed by Darwin, and presented in his book “Origin of Species” published in 1859. The Darwin came upon this theory 20 years previous to the publication of his book during his travels on the Beagle. One stop on his journey was the Galapagos Islands. Darwin identified 13 species of finches in the Galapagos Islands. This puzzled him because he knew of only one species of this bird on the mainland of South America, 600 miles to the east, where they had all supposedly originated. He observed that the Galapagos species differed from each other in beak shape. He also noted that the beak varieties were associated with diets based on different foods. He concluded that when the original South American finches reached the islands, they dispersed to different environments where they had to adapt to different conditions. Over many generations, they changed anatomically in ways that allowed them to get enough food and survive to reproduce.

His theory has also been tested in the laboratory. Controlled studies on male guppies have demonstrated natural selection at work. Those who had larger tails and had brighter coloration, more often females chose them as mates. Unfortunately, those same characteristics made them more vulnerable to predators. When the guppies were placed in a predator free environment, in just a few generations, the male guppies evolved brighter coloration and larger tails. When predators were reintroduced into the environment, the males went back the other way. They evolved smaller tails and less brilliant colors.

One more example of natural selection was discovered in peppered moths living near industrialized English cities. They have varieties that vary in wing and body coloration from light to dark. During the 19th century, sooty smoke from coal burning furnaces killed the lichen on trees and darkened the bark. When moths landed on these trees, the dark colored ones blended into the bark, and were harder to spot by birds that ate them so they lived long enough to reproduce. Over generations, the environment continued to favor darker moths. As a result, they progressively became more common. By 1900, almost all of the moths in the vicinity of those cities were mostly black. Lately air pollution controls have reduced the amount of pollutants reaching the trees. As a result, lichen has grown back, making trees lighter in color. Now, natural selection favors lighter moths, so they have become the most common. This same phenomenon was also witnessed around Detroit, Michigan.

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