8.22: Laboratory and field experiments enable us to watch evolution in progress.

A fifth line of evidence for the occurrence of evolution comes from multigeneration experiments and observations. People once thought that evolution was too slow a process to be observed in action. However, by choosing the right species—preferably organisms with very short life spans—and designing careful experiments, we can observe and measure evolution as it is happening.

In one clever study, researchers looked at populations of grass on golf courses—a habitat where lawn-mower blades represent a significant source of mortality. All the grass on the golf courses was of the same species, but on the putting greens it was cut very frequently, on the fairways it was cut only occasionally, and in the rough it was almost never cut at all (FIGURE 8-43). Over just a few years, significant changes took place in these different grass populations. The grass plants on the greens came to be short lived, with rapid development to reproductive age and a very high seed output. For the plants in these populations, life was short and reproduction came quickly. (For those in which it did not come quickly, it did not come at all; hence their lack of representation in the population.) Plants on the fairways had slightly slower development and a reduced seed output, while those in the rough had the slowest development and the lowest seed output of all. When plants from each of the habitats were collected and grown in greenhouses under identical conditions, the dramatic differences in growth, development, and life span remained, confirming that the frequencies of the various alleles controlling the traits of life span and reproductive output in the three populations had changed. In other words, evolution had occurred.

One particularly disturbing line of evidence for the occurrence of evolution in nature comes from the evolution of antibiotic-resistant strains of bacteria that cause illness in humans. In the 1940s, when penicillin was first used as a treatment for bacterial infections, it was uniformly effective in killing Staphylococcus aureus. However, some strains of Staphylococcus contain alleles that make them resistant to penicillin. Because penicillin has become such a pervasive toxin in the environment of Staphylococcus, natural selection has led to an increase in the frequency of alleles that make these strains resistant to the drug. As a consequence, humans are increasingly at risk for becoming infected with Staphylococcus and getting diseases such as pneumonia and meningitis. Today, more than 90% of isolated S. aureus strains are resistant to penicillin (FIGURE 8-44). In the 1960s, the antibiotics methicillin and oxacillin had nearly complete effectiveness against Staphylococcus. Today, though, nearly a third of staph infections are resistant to these antibiotics as well. The meaning of such unintentional natural selection “experiments” is clear and consistent: evolution is occurring all around us.

Finally, let’s return to the starvation-resistant fruit flies introduced at the beginning of this chapter. They are an unambiguous demonstration that natural selection can produce dramatic changes in a population, and that it can bring about these changes very quickly. And, perhaps most important, we saw that replicating the same evolutionary process over and over again produced the same predictable results. In the laboratory, on the farm, in the doctor’s office, and in deserts, streams, and forests, evolution is occurring.

Reflecting on both the process and the products of evolution and natural selection, in the final paragraph of The Origin of Species Darwin eloquently wrote: “There is grandeur in this view of life…and that…from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”