Life on Earth is astonishingly diverse: bacteria in boiling hot springs, whales that dive thousands of feet, orchids that mimic insects, and humans who build cities and write books. How did this variety arise? The theory of evolution by natural selection, proposed by Charles Darwin and Alfred Russel Wallace in the nineteenth century, offers the explanation that biologists consider the foundation of modern biology. It describes how populations of living things change over many generations and how new species arise. This explainer describes what evolution is, how natural selection works and the evidence that supports it.
What evolution means
In biology, evolution means change in the inherited characteristics of populations over successive generations. It is not something that happens to an individual during its lifetime. A giraffe does not stretch its neck and pass a longer neck to its offspring. Instead, over many generations, the proportion of individuals with certain inherited traits changes in a population.
Evolution happens through several mechanisms, but the best known is natural selection.
Darwin and Wallace
Charles Darwin traveled around the world from 1831 to 1836 aboard the ship HMS Beagle. He collected fossils, plants and animals and observed how species differed from place to place, notably on the Galápagos Islands. Over the next two decades, he developed his theory of natural selection.
Meanwhile, the naturalist Alfred Russel Wallace, working in Southeast Asia, independently arrived at the same idea. In 1858, their ideas were presented together to the Linnean Society of London. The following year, Darwin published On the Origin of Species, which laid out the evidence in detail.
How natural selection works
Natural selection is based on a few observations and the logical conclusion that follows from them:
- Variation: Individuals in a population differ from one another in traits such as size, color, speed or resistance to disease.
- Inheritance: Many of these traits are passed from parents to offspring.
- More offspring than can survive: Most organisms produce more offspring than the environment can support. Not all of them survive and reproduce.
- Differential survival and reproduction: Individuals with traits that help them survive and reproduce in their environment tend to leave more offspring.
As a result, helpful inherited traits become more common in the population over generations, while harmful ones become less common. This is natural selection. The traits that increase survival and reproduction are called adaptations.
The phrase "survival of the fittest," coined by the philosopher Herbert Spencer, is often used to describe natural selection. In biology, "fitness" does not mean strength or health; it means reproductive success, how many offspring an individual leaves.
An example: peppered moths
A famous example comes from the peppered moth in England. Before the Industrial Revolution, most peppered moths were light-colored, blending in with lichen-covered tree trunks. Dark moths were rare. As industrial pollution blackened tree trunks with soot, dark moths became better camouflaged against birds, while light moths became easier to spot. Over decades, dark moths became much more common in polluted areas. When air pollution decreased in the late twentieth century, light moths became common again.
Where variation comes from
Natural selection needs variation to act upon. Variation comes from:
- Mutations: Changes in DNA, the molecule that carries genetic information. Mutations occur naturally when DNA is copied or because of environmental factors. Most are neutral or harmful, but some are beneficial.
- Sexual reproduction: Offspring receive a new combination of genes from two parents, creating unique individuals.
- Gene flow: When individuals move between populations, they bring new genetic variants.
Darwin did not know about genes or DNA. The work of Gregor Mendel on inheritance in pea plants, rediscovered around 1900, and later the discovery of DNA's structure in 1953, explained how traits are inherited and combined with Darwin's ideas into what is called the modern synthesis.
Other mechanisms of evolution
Natural selection is not the only force that changes populations:
- Genetic drift: Random changes in the frequency of genetic variants, especially strong in small populations. By chance, some variants may disappear or become common regardless of their usefulness.
- Sexual selection: Traits that help individuals attract mates, such as the peacock's tail, can spread even if they do not help survival.
- Gene flow: Movement of genes between populations can introduce new variants and make populations more similar.
How new species form
A species is often defined as a group of organisms that can interbreed and produce fertile offspring. New species form through speciation, usually when populations become separated:
- A population is divided, for example by a mountain range, a river or migration to an island.
- The separated populations experience different environments and mutations.
- Over many generations, natural selection and genetic drift make them increasingly different.
- Eventually, they become so different that they can no longer interbreed, even if they meet again.
Darwin's finches on the Galápagos Islands are a classic example. Different species evolved from a common ancestor, with beaks adapted to different foods, such as seeds, insects or cactus flowers. Biologists Peter and Rosemary Grant have documented measurable changes in finch beaks over just a few generations in response to droughts.
The evidence for evolution
Evolution is supported by many independent lines of evidence.
Fossils
The fossil record shows that life has changed over billions of years. Older rock layers contain simpler organisms, while younger layers contain more complex and diverse forms. Transitional fossils show intermediate forms between groups, such as Tiktaalik, a fish with features of four-legged animals, and Archaeopteryx, with features of both dinosaurs and birds. Fossils also show the evolution of whales from land-dwelling mammals.
Comparative anatomy
Different species share similar structures that suggest common ancestry. The forelimbs of humans, bats, whales and cats have the same basic set of bones, adapted for different uses. These are called homologous structures. Some organisms also have vestigial structures, reduced features with little or no function, like the small hip bones in whales.
Embryology
The embryos of many vertebrates look similar in their early stages, reflecting their shared ancestry.
DNA and molecular biology
All living things use DNA and share the same genetic code. Comparing DNA sequences reveals how closely species are related. Humans share about 98 to 99 percent of their DNA sequence with chimpanzees, our closest living relatives. Genetic relationships closely match those predicted from fossils and anatomy.
Biogeography
The distribution of species around the world reflects evolutionary history. Islands often have unique species related to those on the nearest mainland, and Australia's marsupials evolved in isolation.
Observed evolution
Evolution can be observed directly today. Bacteria evolve resistance to antibiotics, insects evolve resistance to pesticides and viruses change rapidly. Antibiotic resistance is a serious medical problem, and it is a clear example of natural selection in action.
Common misunderstandings
- "Evolution is just a theory." In science, a theory is a well-tested explanation supported by extensive evidence, not a guess.
- "Humans evolved from monkeys." Humans and modern apes share a common ancestor that lived millions of years ago. Modern apes are our evolutionary cousins, not our ancestors.
- "Evolution has a goal." Natural selection has no foresight. It favors traits that work in the current environment.
- "Individuals evolve." Populations evolve over generations; individuals do not.
- "Evolution is always slow." Some changes happen over millions of years, but others, especially in microbes, can happen quickly.
Human evolution in brief
Humans are part of the same evolutionary story as all other life. Fossils and genetic evidence show that the human lineage separated from the lineage leading to chimpanzees and bonobos in Africa roughly 6 to 7 million years ago. Over millions of years, many different human relatives lived, including species of Australopithecus, such as the famous fossil skeleton known as "Lucy," about 3.2 million years old, and early members of the genus Homo.
Our own species, Homo sapiens, appeared in Africa roughly 300,000 years ago and later spread across the world. For much of that time, we shared the planet with other human species, such as Neanderthals in Europe and western Asia and the Denisovans in Asia. Ancient DNA has revealed that early modern humans interbred with both, and many people today carry a small percentage of Neanderthal or Denisovan DNA. Human evolution continues today. For example, the ability of many adults to digest lactose, the sugar in milk, spread in populations with a long history of dairy farming within the last 10,000 years.
Why evolution matters today
Understanding evolution is not only about the past. It is essential for modern medicine, agriculture and conservation. Doctors use evolutionary principles to understand how bacteria become resistant to antibiotics and how viruses change, guiding the design of treatments and vaccines. Farmers and scientists manage the evolution of pests resistant to pesticides and breed crops with useful traits. Conservation biologists use genetic diversity to help endangered species survive, and evolutionary relationships help researchers choose which animal models best reflect human biology.
Questions readers ask
What is natural selection? The process by which inherited traits that help organisms survive and reproduce become more common in a population over generations.
What is an adaptation? An inherited trait that increases an organism's survival or reproduction in its environment.
How do new species form? Usually when populations become separated and evolve differently until they can no longer interbreed.
Can we see evolution happening? Yes. Examples include antibiotic resistance in bacteria and changes in the beaks of Galápagos finches.
Did humans evolve from chimpanzees? No. Humans and chimpanzees share a common ancestor that lived roughly 6 to 7 million years ago.
In short
Evolution is the change in inherited traits of populations over generations. Natural selection, proposed by Darwin and Wallace, explains how variation, inheritance and differences in survival and reproduction lead to adaptations. Mutations and sexual reproduction create variation, while genetic drift, sexual selection and gene flow also shape populations. Over time, separated populations can become new species. Evidence from fossils, anatomy, DNA, biogeography and direct observation all supports evolution, making it the unifying framework of biology.