โ Scribed by Sackman, Douglas Cazaux
No coin nor oath required. For personal study only.
It was, evolutionary biologist Theodosius Dobzhansky wrote in 1937, "probably the best proof of the effectiveness of natural selection yet obtained" (1937: 161). If we were guessing what Dobzhansky had in mind, we might nominate some of the classic examples from evolutionary biology and paleontology. Charles Darwin's tortoises in the Galapagos Islands? Rise and extinction of the dinosaurs? Fossils from the Burgess shale? The answer to all these suggestions is no. Dobzhansky's proof was the evolution of insects resistant to insecticides.
In the early twentieth century, fruit growers in the western United States noticed that, over time, some insecticides "lost" their ability to kill scale insects in orchards. Most entomologists blamed people. Manufacturers produced defective insecticides, they reasoned, or farmers applied legitimate products incorrectly. A few entomologists, however, noticed that their data contradicted this explanation. Insecticides lost their potency in areas where farmers bought the most reliable insecticides and sprayed them most carefully, rather than the reverse. Perhaps, these scientists ventured, some insects carried Mendelian genes for resistance to sprays. But the reason why resistant individuals should be common in heavily sprayed areas remained a mystery for the next two decades (
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