Evolution is both fact and theory

In science, evolution is recognized as both an observed fact and a comprehensive theory. The "fact" of evolution is that living things change over time. These changes have been directly observed in nature and in the laboratory, and they are recorded in the fossil record. The "theory" of evolution is the scientific explanation for how and why these changes happen. It brings together mechanisms like mutation, natural selection, and genetic drift to explain the diversity of life.^{1, 2} In everyday language, people often use the word "theory" to mean a guess or a hunch. But in science, a theory is the highest level of understanding—ranking alongside the germ theory of disease or the theory of gravity.

What "theory" means in science

Science uses specific words to describe different levels of understanding. A hypothesis is a testable idea. A scientific law describes a measurable pattern in nature, such as the law of gravity. A scientific theory is a broad explanation that ties together facts, laws, and tested hypotheses. It doesn't just describe what happens; it explains why it happens.

One of the key features of a scientific theory is that it must be testable—and potentially disprovable. Philosopher Karl Popper called this "falsifiability."³ Evolutionary theory is completely falsifiable. For example, if scientists found fossils of modern mammals in rocks that are billions of years old, or if they discovered a complex animal that didn't use DNA or RNA, the theory would be in serious trouble. However, decades of research across many different fields of science have continually supported evolution rather than disproving it.¹

Evolution as both fact and theory

In 1981, evolutionary biologist Stephen Jay Gould wrote a famous essay clarifying that facts and theories are two different things, not rungs on a ladder of certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts.²

The fact of evolution is supported by massive amounts of data from the fossil record, genetics, and the geographic distribution of species. We know that life has changed over millions of years. The theory of evolution—often called the modern synthesis—is the explanation for how that change occurs. Even if scientists debate the exact details of how evolution happens (such as how fast it happens or which mechanisms are most important in specific situations), the fact that it does happen is not in doubt among scientists.^{1, 2}

How evolution works

Scientists have identified four main mechanisms that cause populations to change over time: mutation, natural selection, genetic drift, and gene flow.

Mutation: A mutation is a change in an organism's DNA sequence. Most mutations have no effect, and some are harmful, but occasionally a mutation provides a useful new trait. Mutations act as the raw material for evolution, creating the genetic variety that other forces act upon. For example, sequencing the human genome has shown that each person is born with about 60 to 100 new mutations that their parents didn't have.^{4, 5}

Natural Selection: Charles Darwin and Alfred Russel Wallace proposed this mechanism in 1858.⁶ Natural selection happens when individuals with certain beneficial traits are more likely to survive and reproduce in a specific environment. Over time, these helpful traits become more common in the population. A famous example is the decades-long study by Peter and Rosemary Grant in the Galápagos Islands, which documented changes in the beak sizes of finches in response to changing weather and food supplies.⁷

Genetic Drift: Unlike natural selection, which is driven by survival advantages, genetic drift is completely random. It happens when certain traits become more or less common purely by chance, like flipping a coin. This effect is especially powerful in small populations and plays a major role in evolution at the molecular level, a concept developed in the 1960s by Motoo Kimura.^{8, 9}

Gene Flow: Gene flow occurs when individuals move between different populations and breed, mixing their genes. This movement helps spread new genetic variations across different groups and keeps populations connected to each other.

The Modern Synthesis

When Charles Darwin proposed natural selection, he didn't know how traits were actually passed down from parents to children. That puzzle was solved when Gregor Mendel's work on genetics was recognized in the early 1900s. In the mid-20th century, scientists brought together Darwin's ideas about natural selection with Mendel's laws of genetics.

Scientists like R.A. Fisher demonstrated mathematically that continuous changes (like height) were completely compatible with genetics.⁸ Other scientists, like Theodosius Dobzhansky, confirmed these ideas with real-world experiments on fruit flies.¹⁰ This combination of genetics, paleontology, and biology is known as the "Modern Synthesis." It forms the foundation of modern evolutionary biology, showing exactly how changes in DNA result in the evolution of whole populations.¹

Observing new species form

One of the strongest pieces of evidence for evolution is that we can actually watch new species form—a process called speciation. This has been documented both in laboratories and in the wild.

In the lab, a famous experiment by Diane Dodd in 1989 showed how adapting to different environments can lead to speciation. She divided fruit flies into two groups and fed one group starch and the other maltose. After several generations, the flies adapted to their new diets. When mixed back together, the "starch flies" preferred to mate with other starch flies, and the "maltose flies" preferred maltose flies, showing the early stages of a specific split.¹¹ Another major study is Richard Lenski's Long-Term Evolution Experiment, which tracked E. coli bacteria for decades. Over time, one population evolved an entirely new ability to process citrate—a trait that normal E. coli do not have.¹²

In nature, scientists have observed similar changes. For example, the apple maggot fly historically laid its eggs on hawthorn fruit. When apples were introduced to North America, some flies began laying eggs on apples instead. Because apples and hawthorns fruit at different times of the year, the two groups of flies stopped mating with each other and are currently evolving into two distinct species.^{13, 14} Another example is mosquitoes in the London Underground tunnels, which have adapted to living underground and biting humans, becoming so genetically different that they can no longer mate with mosquitoes living above ground.¹⁵

The predictive power of evolution

A strong scientific theory doesn't just explain the past; it makes accurate predictions about what we will find in the future. Evolution does this constantly.

In paleontology, scientists used evolutionary theory to predict where they might find a "missing link" fossil between fish and four-legged land animals. Based on the theory, they knew such an animal should have lived about 375 million years ago. They traveled to rocks of that exact age in the Canadian Arctic and discovered Tiktaalik, a fossil with fish-like gills and scales but land-animal-like wrists and a flexible neck.¹⁶

Evolutionary theory also successfully predicted genetic discoveries. Before the chimpanzee genome was mapped, scientists predicted that humans and chimpanzees would share a massive amount of identical DNA due to having a recent common ancestor. When the chimpanzee genome was fully sequenced in 2005, it confirmed that humans and chimps share almost 99% of their DNA.¹⁷

In medicine, evolutionary principles are a matter of life and death. The theory predicts that overusing antibiotics will cause bacteria to evolve resistance. Today, the rise of antibiotic-resistant "superbugs" is a major global health crisis, happening entirely through the mechanisms of mutation and natural selection.¹⁸

Many fields point to the same truth

What makes evolution so robust is that multiple, entirely different fields of science all point to the exact same conclusion. The fossil record shows a clear progression of life over time. Biogeography shows that species are distributed around the world in ways that match the movement of tectonic plates. Genetics reveals that all living things share the same basic DNA code.

For example, humans have 46 chromosomes, while great apes have 48. If humans and apes share a common ancestor, where did the extra pair of chromosomes go? Evolutionary scientists predicted that two ape chromosomes must have fused together in the human lineage. When scientists mapped the human genome, they found exactly that: Human Chromosome 2 has the unmistakable remnants of two fused ape chromosomes, complete with leftover, inactive central structures (centromeres) and ends (telomeres) stuck in the middle.¹⁹

When fossils, genetics, geography, and anatomy all independently confirm the exact same story, it elevates evolution from just a theory to the central organizing principle of all biology.¹