Natural selection

A dark form of the peppered moth was first found in Britain in 1811, and over the next 150 years it spread through the soot-covered woods.

The peppered moth

The peppered moth, Biston betularia, rests on tree trunks during the day. Until the 1800s most peppered moths in Britain were light, speckled grey and white, a form called typica. In 1811 a dark form, carbonaria, was found in Britain for the first time. Over the next 150 years the dark form spread quickly in areas where factory smoke had blackened the trees, while the light form stayed more common in clean countryside. Breeding experiments had shown that the colour depends on a single gene, with one allele for light and another for dark.

In the 1950s Bernard Kettlewell tested the idea that birds were doing the selecting. He marked hundreds of light and dark moths, released them in woods, and two days later tried to catch them again with light traps. In polluted woods a larger share of the dark moths came back; in clean woods a larger share of the light moths did. Moths that matched their background were eaten less often. The same pattern of animals turning dark in polluted places, called industrial melanism, has been seen in other species too.

How the lab works

The lab uses made-up beetles in five shades, numbered 0 (pale) to 4 (black), on bark of one of the same five shades. In each generation every beetle survives with a chance of 1 − pressure × (difference ÷ 4), where the difference is how many shade steps it is from the bark. At 50 percent pressure on sooty bark (shade 4), a black beetle has a difference of 0 and always survives, a mid beetle (shade 2) survives 1 − 0.5 × 2 ÷ 4 = 75 percent of the time, and a pale beetle survives 50 percent of the time. The survivors then breed back up to 100 beetles. Each young beetle takes its parent's shade, except that with the mutation rate as its chance it ends up one shade lighter or darker. The numbers are invented for teaching, but the logic is the same as for the moths.

Try a few runs. With pressure 0, colour does not matter and nothing pushes the population anywhere. With pressure 100 percent and the mutation rate at 0, a population that has no beetles of the right shade can only reach the closest shade it already has; selection can pick only from the variation that is there. Raise the mutation rate a little and new shades keep turning up, so the population can creep step by step towards the bark. Then switch the bark from sooty to pale and see how quickly it turns back.

Finches and bacteria

On the Galápagos island of Daphne Major, Peter and Rosemary Grant and their colleagues have measured finches year after year. Beak depth in the medium ground finch varies and is inherited. After an El Niño brought heavy rain, small soft seeds were plentiful and the large hard seeds that big-beaked birds eat were scarce. Small-beaked birds survived and bred better, and in the following years the average beak size in the population was smaller. When larger seeds came back from 1987, the trend stopped. No bird's beak changed during its life; the average changed because of who left young.

Bacteria show the same thing much faster. When an antibiotic is used, bacteria without a resistance gene die and the few with it survive and divide, so the population becomes resistant. Experiments have shown that the antibiotic does not cause the resistance mutations; the variation was already there, often at a low frequency. Evolution has no goal either. A trait that fits today can be a problem tomorrow if the surroundings change, and natural selection only favours what works in the place and time an organism actually lives. Luck matters too: a perfectly camouflaged moth can still be hit by a falling branch. Selection is a matter of averages over many individuals.