How Does Evolution Actually Work?

Evolution works through a simple loop repeated over many generations. Living things vary, partly because their DNA changes and gets shuffled. Some of those differences help individuals survive and reproduce, so those traits become more common. Chance also nudges which traits spread. Over long stretches of time, these small shifts add up, and populations can split into new species. Every living thing on Earth is linked by this process to shared ancestors.

This is established science. It is backed by fossils, by DNA and by evolution scientists have watched happen in real time. Researchers still argue about details, but not about whether it happens.

Where does new variation come from?

Evolution needs differences to work with. According to the University of California, Berkeley’s Understanding Evolution project, there are three main sources.

• Mutation is a change in the DNA sequence. It can create a brand-new version of a gene. A single mutation can have a big effect, but evolutionary change often builds up from many small ones.
• Recombination happens through sexual reproduction. It shuffles existing genes into new combinations, a bit like reshuffling a deck of cards. No new cards are made, but new hands appear.
• Gene flow is the movement of genes from one population to another, for example when animals migrate.

What do natural selection and chance do with it?

Natural selection is the idea Charles Darwin and Alfred Russel Wallace both reached independently. Their work was read together at the Linnean Society in London in July 1858, and Darwin’s On the Origin of Species followed in November 1859, as the Darwin Correspondence Project records.

Berkeley’s classic example uses beetles. Some are green and some are brown. Birds spot and eat the green ones more often. Brown beetles survive and have more young, and color is inherited, so the population slowly turns brown. If you have variation, differences in reproduction, and inheritance, you get evolution by natural selection.

You can see the same thing in hospitals. When antibiotics kill most bacteria in an infection, the few germs that can resist the drug survive and multiply. The U.S. Centers for Disease Control and Prevention reports more than 2.8 million antimicrobial-resistant infections and more than 35,000 deaths in the United States each year. That is evolution with real costs.

Not every change is driven by selection. Genetic drift is change by pure luck. In Berkeley’s beetle example, someone happens to step on a few green beetles. The population shifts, not because brown is better, but because of a random accident.

How do we know all life is related?

The idea that all living things share ancestors is called common descent. Several independent lines of evidence point the same way.

Fossils. In 2004, researchers in Arctic Canada found Tiktaalik, a creature that lived about 375 million years ago. It had scales and fins like a fish, but its front fins held the bones of an arm, including an elbow and a wrist. The team did not stumble on it by luck. They went looking in Arctic rocks of the right age, laid down in shallow water, where a fish-to-land-animal link ought to be. That is evolution making a prediction, and the rocks answering.

DNA. Our genes carry a family history. The Smithsonian’s Human Origins Program notes that in the parts of the genome compared directly, human and chimpanzee DNA differ by about 1.2 percent, with another 4 to 5 percent of difference when larger insertions and deletions are counted. DNA shows our lineages split from a common ancestor that lived between 8 and 6 million years ago.

Evolution in action. In 1988, biologist Richard Lenski started 12 flasks of E. coli bacteria from a single ancestor. The flasks contained citrate, a nutrient normal E. coli cannot use when oxygen is present. For more than 30,000 generations, no population evolved a way to eat it. Then one did, as Lenski’s team reported in 2008. Their work showed the new ability depended on at least one earlier “potentiating” mutation in that population’s history. The experiment reached 75,000 generations before moving to the University of Texas at Austin in 2022, where it continues.

Isn’t evolution “just a theory”? Did we come from monkeys?

In everyday speech, “theory” means a hunch. In science it means something much stronger. The U.S. National Academies define a scientific theory as a well-substantiated explanation of some part of the natural world, one that ties together many facts and can predict things not yet observed. Gravity is also explained by a theory.

And no, humans did not evolve from monkeys or chimpanzees. As the Smithsonian puts it, we did not evolve directly from any primate living today. Humans and chimps are more like cousins who share great-grandparents than like grandchildren and grandparents.

Some people assume that accepting evolution means giving up faith. Many religious people do not see it that way. The National Academies note that many religious denominations accept evolution. They cite Pope John Paul II, who said in 1996 that evolution is more than a hypothesis, and more than 10,000 Christian clergy who signed a letter saying science and faith can coexist. The Pew Research Center has found that religious Americans’ answers depend partly on how the question is asked, and that nearly all white evangelical Protestants who accept human evolution say God had a role in it. Whether a purpose or a Creator lies behind nature is a question of philosophy and belief. Science can inform it but cannot settle it. We explore that more in Science and Faith.

What do scientists still not know?

Plenty. Biologists debate how much of evolution is driven by selection and how much by drift. They study how new abilities, like eating citrate, depend on earlier chance events. Even the Lenski bacteria keep surprising researchers: after 75,000 generations, they were still slowly improving. The fossil record also has gaps, because fossils form only under special conditions.

Evolution also has limits as an explanation. It describes how life changes once it exists. It does not explain how the very first life began. That separate puzzle is covered in How Did Life on Earth Begin?

What would change the answer?

Science stays open to evidence, even for its strongest ideas. The big picture would be in serious trouble if, for example, fossils were found in the wrong order, such as mammals in rocks far older than the first fish. It would also struggle if DNA family trees and fossil family trees kept pointing in completely different directions. After more than 160 years of looking, the evidence keeps fitting together instead. What changes, year to year, is our understanding of the details.

Evolution is a story written in rocks, in genes and in flasks on a lab shelf. If it raises new questions for you about life, meaning or faith, that is a good sign. Keep asking.