Evolutionary History: Prospectus for a New Field

Edmund Russell

IT WAS, evolutionary biologist Theodosius Dobzhansky wrote in 1937, "probably the best proof of the effectiveness of natural selection yet obtained."1 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? The 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.	1
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.2	2
In the 1930s, Dobzhansky solved this puzzle by discarding an unspoken assumption. Entomologists had assumed that insect species stayed constant (or evolved so little that change was trivial) in historical time. Dobzhansky suggested the opposite. Spraying was a form of natural selection, he argued. By chance, a few individuals within a species carried genes conferring resistance to insecticides. Insecticides killed off susceptible individuals and left the resistant ones behind to reproduce. Resistant individuals passed on their genes for resistance to their offspring. Every time farmers sprayed, they increased the ratio of resistant to susceptible insects in their orchards. Eventually, so few susceptible individuals remained that insecticides appeared to have lost their potency. This deceptively simple explanation solved a pair of biological puzzles: why genes for resistance should become more common over time, and why resistance should be most common in areas sprayed most heavily. It also solved an economic puzzle: why crop losses to insects should rise despite ever-greater doses of insecticides. Further, it provided the paradigmatic example for a new approach to biology, the modern (neo-Darwinian) synthesis that united evolutionary theory with genetics.3	3
Dobzhansky's findings challenged several ideas about evolution that remain common today. Many of us think of evolution as something that happened in the distant past, took eons to occur, and was done by nature. Resistant insects, however, evolved recently rather than long ago, quickly rather than over eons, and under the influence of humans rather than nature alone. Many of us think of evolution as speciation, but populations of insects evolved resistance without budding into new species. Many of us think of a species' genes as fixed in historical time and space, or as varying so little that differences are trivial. But members of insect species carried different versions of genes before insecticides arrived, and spraying increased the proportion of genes for resistance in certain parts of species' ranges enough that resistance became an economic problem. Many of us think of evolutionary ideas as tools for biologists, not humanists. But humans have shaped the evolution of countless species for millennia, reshaping human experience as well as the genes of other species.4	. . .