Selection

An adaptation is a feature that helps a living thing survive in its environment: a camel’s water-saving body, a polar bear’s white fur, a cactus’s spines, a finch’s beak shaped for its food. Living things in the same place compete for food, water, space, light and mates.

Natural selection, step by step:

1. Variation. Individuals in a population differ, because of the genes they inherited.
2. Competition. There is not enough food, space or safety for everyone.
3. Survival. The individuals best suited to the environment are more likely to survive.
4. Breeding. The survivors breed and pass their useful features on to their offspring.
5. Change. Over many generations the useful feature becomes more common. The population has changed: that is evolution.

The peppered moth. Peppered moths rest on tree bark by day and come in a pale speckled form and a dark form. In the 1800s, soot from factories blackened the trees in towns such as Manchester. Birds could see the pale moths against the dark bark and ate them; the dark moths survived and bred. The first dark moth was recorded in Manchester in 1848, and by 1895 about 98% of the city’s peppered moths were dark. After the Clean Air Act of 1956 the bark lightened, and the pale form came back. The soot did not make moths dark. It made being dark an advantage.

“Survival of the fittest” means survival of the best fitted to the environment, not the strongest. A change in the environment, such as soot, a drought or a new food, changes which variation is best fitted. Selection works on a whole population over many generations. An individual cannot evolve in its lifetime, and a giraffe stretching its neck does not give its babies longer necks: giraffes that happened to have longer necks survived and bred more.

Selective breeding. People can do the selecting too. Farmers and breeders choose the parents with the feature they want, such as cows that give the most milk, wheat with big grains or dogs with a gentle nature, breed from them, and repeat it generation after generation. Every breed of dog was bred from wolves this way, and every dog breed is still one species. It is the same idea as natural selection (sometimes called artificial selection), but people choose instead of the environment.

Who worked it out. Charles Darwin collected finches and other animals on the Galápagos Islands in 1835; the beaks on each island suited the local food. Alfred Russel Wallace, collecting in the islands of South-East Asia, came to the same idea on his own and wrote to Darwin in 1858. Their work was presented together in London that year, and Darwin published On the Origin of Species in 1859.

Stretch Bacteria can become resistant to antibiotics by natural selection, very fast, because they breed so quickly: any that happen to survive a drug breed and spread. It is one reason to take antibiotics exactly as the doctor says. Earlier, Jean-Baptiste Lamarck had suggested that animals pass on changes they make during their lives; that idea was wrong.