Darwin, Wallace and the Theory of Evolution

He saw that individuals of a species differ from each other, and he studied how pigeon breeders, by choosing the birds they bred from, made new varieties in a few generations.

On the islands, which lie off the coast of Ecuador, he found birds, tortoises and mockingbirds that were similar but differed from island to island. The finches had beaks suited to the food available on each island.

In South America he found the fossils of giant extinct animals that were like the living armadillos and sloths of the same continent, so the living species seemed to be related to the extinct ones.

Malthus's essay made him realise that animals produce more young than the food supply can support, so many must die.

Organisms produce more offspring than can survive, but the size of a population stays about the same, so there must be a struggle for food, space and mates.

Individuals in a species vary, and some of the variation is passed on from parents to their offspring.

The individuals whose variations are best suited to the environment are more likely to survive and to breed, so they pass on their features to more offspring.

Over many generations the features that help survival become more common, and a species can change into a new one. All species are therefore related through common ancestors.

Wallace collected animals and plants in the Amazon from 1848 to 1852, and in the islands of South East Asia from 1854 to 1862. He studied how species differ from one island to the next.

In 1858, while ill with a fever on the island of Ternate, he thought of natural selection. He wrote it down in an essay and sent it to Darwin, whom he knew, to ask what he thought.

Wallace's studies of the distribution of species led him to draw a line between the animals of Asia and those of Australia. It is called the Wallace Line, and it is still used.

Darwin had been working on the same idea for twenty years but had not published it. His friends arranged for the two men's papers to be read together at a meeting of the Linnean Society of London on 1 July 1858.

Many people, including many scientists, believed that God had created every species separately, as the Bible describes. The theory seemed to contradict this and implied that humans are related to other animals.

Darwin could not explain how features were passed on, or where new variations came from, because genes and mutations were not known. Many thought that the features of parents blended in their offspring.

The changes are too slow to be seen in a lifetime, and the fossil record had gaps, so there was little direct evidence.

Some scientists favoured earlier explanations, such as Lamarck's idea that features acquired in life could be inherited.

Species are now grouped by how closely they are related by evolution, and DNA sequences are used to make evolutionary trees.

Understanding natural selection explains why bacteria become resistant to antibiotics, why the flu virus changes each year so that new vaccines are needed, and why cancer cells can become resistant to drugs.

Pests and weeds evolve resistance to pesticides and herbicides, so farmers rotate chemicals and crops. Selective breeding also depends on the variation that Darwin described.

The theory led to a search for how variation is inherited, and so helped the development of genetics and the study of the genome.

Species need genetic variation to adapt to change, so conservation aims to protect it, for example in seedbanks.

A seedbank is a store of seeds, kept cold and dry. The Millennium Seed Bank in West Sussex stores wild plant seeds from tens of thousands of species, and the Svalbard Global Seed Vault in Norway keeps back-up copies of over a million samples of crop varieties.

The seeds are dried to a very low water content and frozen at about minus 18 to minus 20 degrees Celsius. Respiration and germination almost stop, so that many seeds stay alive for decades or even centuries.

They are an insurance. If a species or a variety of a crop is lost to disease, climate change, habitat destruction or war, the seeds can be grown again to restore it.

A stored variety may have an allele, such as one for resistance to a disease or to drought, that breeders or genetic engineers will need in the future.