How does evolution work? Natural selection, explained

Evolution is the change in the inherited traits of a population of living things over many generations. What does most of the work is natural selection: individuals vary, some of that variation is passed on in their genes, and those whose traits help them survive and reproduce leave more offspring, so their traits become more common. Chance and isolation add to it, and over millions of years the result is new species.

In short

• Natural selection needs three things: variation, inheritance, and some variants leaving more offspring than others.
• Populations evolve, not individuals. No creature changes its genes by trying; the mix of genes in the next generation shifts.
• The evidence comes from fossils, DNA, and evolution watched as it happens, in bacteria, insects and birds.

How does natural selection work, step by step?

Charles Darwin and Alfred Russel Wallace worked out the idea independently, and their papers were read together in London in 1858. Darwin's On the Origin of Species followed in 1859. Their logic, set out in the same three steps by the University of California's Understanding Evolution project, is simple enough to state in a sentence each.

First, variation: individuals in a population differ. Some beetles are green and some brown; some finches have deeper beaks than others. Second, inheritance: some of those differences are written in DNA and passed from parents to offspring. Third, differential reproduction: in a given environment, some variants survive and breed more successfully than others. Where all three hold, the population changes. There is no goal or plan in this, only a sorting of what already varies.

New variation comes from mutation, small changes in DNA when it is copied, and from the reshuffling of genes in sexual reproduction. Most mutations do nothing noticeable, some are harmful, and a few happen to help. Selection doesn't create mutations to order; it acts on whatever turns up.

What does selection look like with real numbers?

Take a population of 1,000 beetles living on brown soil. At the start, 100 are brown and 900 are green, so brown beetles make up 10%. Birds spot green beetles more easily, and suppose that, on average, each brown beetle leaves 1.2 surviving offspring for every 1 that a green beetle leaves: a 20% advantage. Color is inherited.

Each generation, the ratio of brown to green beetles is multiplied by 1.2. It starts at 1 to 9. The brown share then runs like this:

Generation	Brown share of the population
0	10%
5	22%
10	41%
20	81%
30	96%

In 30 generations, the rare color has become the normal one, without a single beetle changing its own color. Selection changes populations, not individuals. Now shrink the advantage to 1%, the kind of tiny edge no observer would notice in a single season. Brown beetles still take over; it just takes longer. Going from 10% to 50% now takes about 220 generations instead of about 12. For insects that breed every year, that is a couple of centuries, an eyeblink in the history of life.

Is evolution only natural selection?

No. Selection is the process that shapes adaptations, but it isn't the only way populations change.

Genetic drift is change by chance. In a small population, a storm might kill a few individuals at random, and the genes they carried vanish whatever their worth. Drift matters most when populations are small, and it can fix or erase traits that make no difference to survival. Gene flow is the movement of genes between populations as individuals migrate and interbreed, which keeps populations alike. Mutation supplies the raw material for all of it.

Speciation, the origin of new species, often starts when a population is split, by a mountain range, a river changing course, or a few individuals reaching an island. Cut off from each other, the two groups collect different mutations, drift apart and adapt to different conditions. Given enough time, they can no longer interbreed even if they meet again. That's how one ancestral finch species gave rise to the more than a dozen species on the Galápagos Islands.

What is the evidence for evolution?

Three independent kinds of evidence point the same way. That is why, when the U.S. National Academy of Sciences published its 2008 report Science, Evolution, and Creationism, the biologist Francisco Ayala wrote in its journal that biological evolution is "the central organizing principle of modern biology", one of the best-tested ideas in science.

Fossils show life changing over time in a consistent order: single cells alone for billions of years, then simple animals, then fish, then four-legged land animals, and later mammals and birds. Transitional forms keep turning up where predicted, such as Tiktaalik, a fish with features of land animals found in rocks about 375 million years old, and feathered dinosaurs linking dinosaurs to birds. Mass extinctions, like the one that killed the dinosaurs, show how the survivors then spread into the empty space.

DNA gives a second, separate family tree, and it matches the one from fossils and anatomy. Species thought to be close relatives share more of their genetic code. According to the Smithsonian's Human Origins Program, human and chimpanzee DNA differ by about 1.2% in base substitutions in shared genes, with a further 4 to 5% difference from segments deleted, duplicated or inserted.

Evolution observed is the third. Since 1988, Richard Lenski's team at Michigan State University has grown 12 populations of the bacterium E. coli, passing well over 65,000 generations by 2016. They watched the bacteria grow fitter in their flasks, and one population evolved the ability to feed on citrate, which E. coli normally can't use in the presence of oxygen. Outside the lab, bacteria evolve resistance to antibiotics, the same selection at work in bacteria and viruses that makes resistance a medical problem. On the Galápagos island of Daphne Major, biologists Peter and Rosemary Grant measured finch beaks getting deeper after a drought in 1977 left mostly large, hard seeds.

What do people get wrong about evolution?

"Survival of the fittest" means the strongest win. "Fittest" means best suited to reproduce in a particular environment, which can mean small, well hidden or quick to breed. A trait that helps in one place can hurt in another. The phrase was Herbert Spencer's, which Darwin adopted in later editions.

Humans evolved from chimpanzees. Humans and chimpanzees share a common ancestor that lived roughly 6 to 8 million years ago. Both lineages have been evolving ever since; neither is a stage on the way to the other.

Evolution is "just a theory". In science a theory is a well-tested explanation that ties many facts together, as in the theory of gravity, not a guess.

Evolution aims at progress or complexity. It has no direction. Parasites often evolve to be simpler, and cave fish lose their eyes when eyes stop being worth their cost.

Evolution and religious belief are always at odds. Many religious people and denominations accept evolution, seeing science as explaining how life changed and faith as answering other questions. Others hold to creationism, the belief that species were created in their present form, for religious reasons. The science described here is about evidence from nature, and doesn't settle questions of faith.

Check yourself

• Without looking back: name the three conditions natural selection needs, with an example of each.
• Why does even a 1% advantage eventually change a population, and what decides how long it takes?
• What is the difference between natural selection and genetic drift?

Sources

• Natural selection, Evolution 101 — Understanding Evolution, University of California Museum of Paleontology
• Science, evolution, and creationism — Francisco J. Ayala, Proceedings of the National Academy of Sciences, 2008
• Genetics, Human Evolution Evidence — Smithsonian National Museum of Natural History
• A Conductor of Evolution's Subtle Symphony — Stephanie Bucklin, Quanta Magazine, 2016
• Evolution — Wikipedia