The geneticist Theodosius Dobzhansky once argued, in a 1973 essay, that nothing in biology makes sense except in the light of evolution. This level explains why biologists see evolution as the idea that ties their whole science together.

Before Darwin

The idea that species change over time is older than Darwin. In 1809 the French naturalist Jean-Baptiste Lamarck proposed that traits acquired during an animal's life are passed to its offspring. On this view, giraffes stretching for high leaves would lengthen their necks a little and pass longer necks to their young. Lamarck deserves credit for treating species as changeable, but experiments did not support his mechanism. A weightlifter's children are not born with bigger muscles.

Darwin and Wallace

Charles Darwin (1809 to 1882) sailed around the world on HMS Beagle from 1831 to 1836. On the Galápagos Islands and elsewhere, he noticed that species varied from place to place in ways that suggested common descent. For two decades he gathered evidence. Then in 1858 he received a letter from the naturalist Alfred Russel Wallace, working in what is now Malaysia and Indonesia, who had independently reached the same idea. Their papers were presented together in London in 1858, and Darwin published On the Origin of Species in 1859.

How natural selection works

The argument rests on a few observable facts:

1. Variation. Individuals in a population differ.
2. Inheritance. Many of those differences are passed from parents to offspring.
3. Overproduction. More offspring are born than can survive.
4. Differential success. Individuals with traits suited to their environment tend to survive and reproduce more.

Over generations, helpful traits become more common. Populations evolve; individuals do not. In evolution, fitness means success at surviving and reproducing, not strength or speed. The phrase “survival of the fittest” was coined by the philosopher Herbert Spencer in 1864, not by Darwin, though Darwin later adopted it.

Natural selection is not the only mechanism. Genetic drift is random change in how common alleles are, which is strongest in small populations. If a storm happens to kill most of a small island's lizards, the survivors' genes spread by luck, not merit. Mutations add new variation, and migrants carry genes between populations.

The evidence

• Fossils appear in a consistent order in rock layers, and transitional forms link major groups. Tiktaalik, found in Arctic Canada in 2004, is a roughly 375-million-year-old fish with a neck, sturdy fins and other features of limbed animals.
• Anatomy. A human arm, a bat's wing and a whale's flipper are built from the same set of bones arranged differently, which makes sense if they share an ancestor.
• DNA. Genetic comparisons confirm the family tree built from anatomy. Humans and chimpanzees are close relatives: we did not descend from chimpanzees, but we share a common ancestor that lived roughly 6 to 7 million years ago.
• Direct observation. On the Galápagos island of Daphne Major, the biologists Peter and Rosemary Grant measured finches for decades. During a drought in 1977, small soft seeds ran out and mostly large hard seeds remained. Finches with deeper beaks survived better, and in the next generation the average beak depth increased.

Common misunderstandings

Evolution has no goal. Organisms do not evolve because they need to; variation arises by chance, and the environment favors some of it. Nor is evolution “just a theory” in the everyday sense: as Level 1 explained, a scientific theory is a well-tested explanation.

Evolution and society

The scientific consensus on evolution is overwhelming, and many religious believers accept it; in 1996 Pope John Paul II described it as more than a hypothesis. Others reject it on religious grounds, a dispute that reached an American courtroom in the Scopes trial of 1925. Students can understand the science while making up their own minds on deeper questions of meaning.

A more troubling legacy is Social Darwinism, which used “survival of the fittest” to justify poverty, colonialism and racism. Its basic error is to treat a description of what happens in nature as a rule for how people ought to treat one another. Darwin himself closed the Origin with wonder rather than cruelty: “from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

Worked example: when antibiotics stop working

A patient has a bacterial infection with billions of bacteria. Watch natural selection happen in weeks rather than millennia.

1. Variation. By chance mutation, a few bacteria carry a gene that lets them survive the antibiotic.
2. Selection. The drug kills most bacteria. The resistant few survive.
3. Inheritance. They divide, passing on the resistance gene. Bacteria can also share genes with each other directly.
4. Result. The new population is mostly resistant. The drug no longer works.

No bacterium decided to become resistant. The drug simply changed which bacteria survived. This is why doctors warn against antibiotics for viral illnesses such as colds, where they do nothing to the virus but give resistant bacteria an advantage, and why overuse in medicine and farming worries public health experts.