Like any scientific theory, the theory of evolution by natural selection can be tested by experiment. Since the publication of On the Origin of Species in 1859, biologists have devised many ingenious ways to examine the effect of natural selection on living organisms. This article looks at three such studies.
In the late 1970s, American zoologist John Endler tested Darwin’s theory using a popular species of aquarium fish, the guppy (Poecilia reticulata). Male guppies have brightly colored spots to attract females, but these spots also attract predators. It had previously been observed that males living in streams where there were many predatory fish tended to have fewer spots, which lessened their risk of being eaten, whereas those who lived in streams with fewer predators had more spots.
To replicate this effect experimentally, Endler placed groups of male and female guppies in three ponds that were identical except that one pond contained no predators, one contained a species of predatory fish that preys specifically on guppies, and one contained a different predatory fish that does not feed on guppies.
After leaving the guppies to breed for 20 months (representing several generations for the fish, which begin reproducing when they are about 3 months old), Endler found that the males in the ponds containing no predators or predators that do not eat guppies had significantly more spots than the males that shared their pond with a guppy predator. As male guppies’ coloration is inherited from their fathers, this experiment provides strong evidence that spot number in guppies has evolved as a “trade-off” between the need to attract mates and the need to avoid being eaten.
Ecotypic Variation in the Dominican Anole
While Endler’s guppies show how natural selection can lead to a marked change in a character that affects the survival and reproduction of a population, recent research on a lizard, the Dominican anole (Anolis oculatus), illustrates one way in which new species might arise.
In this experiment, researchers placed similar groups of anoles in secure enclosures located in a variety of island habitats ranging from dry coastal woodland to mountain rainforest. When they later measured characters that had earlier been shown to be at least partly inherited, such as leg and toe length, the width of the head and scale color and shape, they found that these features now varied between the different lizard populations in a pattern that depended on their habitat.
This finding is especially interesting because it hints at how speciation might occur in nature. If the experimental groups of A. oculatus had been kept in isolation for many more generations, the differences between them might have eventually become so great that zoologists would classify them as separate species.
Indeed, splitting of species due to geographical separation – a process known as “allopatric speciation” – appears to have happened already among anole lizards in the Caribbean, where different islands, with different habitats, are each home to different species.
Industrial Melanism in the Peppered Moth
Perhaps the most famous example of evolution in action is the case of the peppered moth (Biston betularia), which illustrates the concept of “selection pressure” – the force that drives evolution.
The British peppered moth feeds at night and spends the day resting on tree trunks, where it is at risk of being eaten by birds. Until the mid-1890s, all peppered moths had a pale, speckled coloration that provided camouflage against the pale lichen that covered the bark of their trees. In the latter half of the 19th century, however, a black form of the moth was first observed, and by 1900 almost all peppered moths in urban areas were black, whereas most of those in rural areas remained pale.
This 50-year period coincided with the rise of industrialization in British cities, and it was suggested that the change in moth color in urban areas was due to sulfur dioxide fumes from factories killing the lichen on tree trunks. The dark bark beneath was good camouflage for black peppered moths, but pale moths were now at a disadvantage because they were more easily spotted by predatory birds.
In 1956, the entomologist H. Bernard Kettlewell set about examining this hypothesis by collecting, marking and releasing both black and pale peppered moths in Birmingham (an urban area) and Dorset (a rural area). He then laid more traps to recapture the marked moths, and observed that a smaller proportion of the pale form was recaptured in Birmingham and a smaller proportion of the dark form in Dorset. This finding was consistent with the suggestion that pale moths were more conspicuous – and therefore easier prey for birds – in areas with dark, polluted trees, whereas dark moths were more likely to be eaten in rural areas with pale, lichen-covered trees. It could be concluded that the selection pressure of bird predation had driven the evolution of two different forms of B. betularia.
Kettlewell’s experiment has been repeated, and his conclusion confirmed, several times, most recently in 2003. Since the Clean Air Act of 1956, however, emissions of sulfur dioxide have decreased in the UK, and these later studies have documented the decline of the dark peppered moth in industrial regions. By 1985, it had largely disappeared from the Midlands and was found in high numbers only in the far North East of England.
Short-lived Species Provide Evidence of Natural Selection
It might be difficult to observe the evolution of long-lived animals such as tortoises, elephants and humans, but, as these three experiments show, fish, insects and other species with short generation times are ideal subjects for the study of natural selection in action.
Butler, D., Gillman, M., Metcalfe, J., Robinson, D. Life. Milton Keynes: The Open University, 2008.
Cook, L. The Rise and Fall of the Melanic Peppered Moth. Accessed 06-07-11.
Endler, J. Natural Selection on Color Patterns in Poecilia reticulata. Evolution. Accessed 06-07-11.
Thorpe, R., Reardon, J., Malhotra, A. Common Garden and Natural Selection Experiments Support Ecotypic Differentiation in the Dominican Anole (Anolis oculatus). The American Naturalist. Accessed 06-07-11.