ScienceVerseNatural selection
Natural selection is the process by which individuals whose inherited traits suit their environment tend to survive and reproduce more, so those traits become more common in the population over generations. Charles Darwin and Alfred Russel Wallace proposed it independently in 1858, and it is the only known mechanism of adaptive evolution. It has been measured directly in the wild — in Darwin's finches, in peppered moths, and in bacteria that become resistant to antibiotics.
Measuring selection in the wild
Long-term field studies quantify selection directly. On Daphne Major, an El Niño period that left mainly small, soft seeds favoured small-billed medium ground finches; because bill size is heritable, mean bill size fell in following generations, and the trend stopped when larger seeds returned in 1987. On an undisturbed Galápagos island, 22 years after a competitor (Geospiza magnirostris) arrived, G. fortis diverged in beak size when the two species severely depleted the food supply together — character displacement observed as it happened, and the strongest evolutionary response recorded in 33 years of study.
In the peppered moth, Majerus's six-year release of 4,864 moths found daily selection against the melanic form of about 0.1, sufficient in magnitude and direction to explain its rapid post-industrial decline.
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What natural selection is
Natural selection is not a force that designs living things. It is what happens automatically when three conditions hold together in a population. Darwin argued that, given these conditions, selection is inevitable.
- Inheritance: most characteristics are passed from parents to offspring.
- Overproduction: more offspring are produced than can survive, so food, space and mates are limited.
- Variation: offspring differ from one another in inherited characteristics.
Individuals whose inherited variations help them compete for limited resources leave more offspring, so those variations become better represented in the next generation. Over many generations this adapts the population to its local environment. Biologists measure success with fitness: an individual's contribution to the next generation's gene pool, judged relative to others in the same population.
Three patterns of selection
- Stabilising selection favours the average and removes extremes — for example, mice whose coats best match a uniform forest floor.
- Directional selection favours one end of the existing range, often after the environment changes.
- Diversifying selection favours two or more extremes over the intermediate form.
Worked example (hypothetical)
This example uses made-up numbers to show the logic. A beetle population has 50 green and 50 brown beetles living on brown soil, and colour is inherited. Birds spot green beetles more easily, so suppose 30 green and 45 brown beetles survive to breed. Brown beetles were 50% of the population before; among the survivors they are 45 ÷ 75 = 60%. If each survivor leaves the same number of offspring, the next generation starts at about 60% brown. Repeated over many generations, the population shifts towards brown — no beetle changed colour.
Where this connects
Natural selection needs inheritance and the variation that alleles of genes supply, and it is the main mechanism that explains evolution — the change of populations over generations.
How we know
Peter and Rosemary Grant measured medium ground finches on the Galápagos island of Daphne Major for decades. After an El Niño period left few large, hard seeds but plenty of small, soft ones, small-billed birds survived and bred more, and the average bill size of the population fell; the trend stopped when larger seeds returned in 1987. Their study from 1972 to 2001 found selection happening often, sometimes reversing direction, so that the finches' long-term path could not have been predicted. In a later study on an undisturbed Galápagos island, medium ground finches diverged in beak size from a competitor species 22 years after the competitor arrived, once the two together had severely depleted the food supply.
Dark (melanic) peppered moths became common in Britain during the industrial revolution and have declined rapidly since 1970. In Michael Majerus's six-year experiment, 4,864 moths were released and birds ate the dark form more often, at a rate large enough to explain the decline — strong direct evidence that camouflage and bird predation drive the change.
Selection we can watch: antibiotic resistance
Antimicrobial resistance is a naturally occurring process. When bacteria or fungi are exposed to a drug, the resistant ones survive, multiply and spread. The resistance was already there; the drug does not create it. The World Health Organization identifies the misuse and overuse of antimicrobials as what is driving the development and spread of drug-resistant pathogens.
Assumptions and limits
- Selection changes the next generation only if the favoured trait is heritable; survivors that differ for non-inherited reasons leave no lasting change.
- Fitness is relative: what counts is reproductive contribution compared with others in the same population, in that environment.
- Short-term change can be predicted from selection and inheritance, but long-term outcomes cannot, because the environment that sets selection's direction itself fluctuates.
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Sources and methodology
- Darwin argued that natural selection follows inevitably from three principles: most characteristics are inherited, more offspring are produced than are able to survive so resources are limited, and offspring vary in their inherited characteristics. (awaiting scientific review)
- Charles Darwin and Alfred Russel Wallace independently conceived natural selection and presented papers on it at the Linnean Society in London in 1858; Darwin's On the Origin of Species was published the following year. (awaiting scientific review)
- Natural selection leads to greater adaptation of a population to its local environment and is the only known mechanism of adaptive evolution. (awaiting scientific review)
- The variation that natural selection acts on is already present in a population and does not arise in response to an environmental change; for example, antibiotics select resistant bacteria whose resistance did not arise because the antibiotic was applied. (awaiting scientific review)
- In the early nineteenth century Jean-Baptiste Lamarck proposed that species change through the inheritance of acquired characteristics — modifications an individual gains during its life — a mechanism that many discredited, although his ideas were an important influence on evolutionary thought. (awaiting scientific review)
- Evolutionary (Darwinian) fitness is an individual's contribution to the gene pool of the next generation, and what matters for natural selection is relative fitness compared with other individuals in the population. (awaiting scientific review)
- Stabilising selection favours an average phenotype, directional selection favours phenotypes at one end of the existing range of variation, and diversifying selection favours two or more extreme phenotypes over intermediates. (awaiting scientific review)
- On the Galápagos island of Daphne Major, after an El Niño period left few large hard seeds but many small soft seeds, the average bill size of medium ground finches became smaller because small-billed birds survived and reproduced more; the trend stopped when larger seeds became more available in 1987. (awaiting scientific review)
- A study of two Darwin's finch species on Daphne Major from 1972 to 2001 found that natural selection occurred frequently, body size and two beak traits changed several times, and the species' states at the end could not have been predicted at the start. (awaiting scientific review)
- Unpredictable evolution in a 30-year study of Darwin's finches (Grant & Grant) — Peer-reviewed paper
- On an undisturbed Galápagos island, medium ground finches (Geospiza fortis) diverged in beak size from a competitor species, Geospiza magnirostris, 22 years after the competitor arrived, when the two species jointly and severely depleted the food supply; the evolutionary response was the strongest recorded in 33 years of study. (awaiting scientific review)
- Evolution of character displacement in Darwin's finches (Grant & Grant) — Peer-reviewed paper
- Melanic (dark) peppered moths became common in Britain during the industrial revolution, and since 1970 their frequency has fallen rapidly. (awaiting scientific review)
- The classical explanation of peppered-moth melanism — birds preying more on whichever form is less camouflaged — was attacked in the years before 2012, and there was general scepticism about birds as the selective agents. (awaiting scientific review)
- In Michael Majerus's six-year experiment releasing 4,864 peppered moths — the largest such experiment ever attempted, analysed and published by colleagues after his death — birds preyed more heavily on the melanic form, with daily selection against melanics of about 0.1, enough to explain their recent rapid decline in post-industrial Britain. (awaiting scientific review)
- Antimicrobial resistance is a naturally occurring process, but its increase is driven by germs being exposed to antibiotics and antifungals and by the spread of those germs and their resistance mechanisms; the resistant germs survive, multiply and spread. (awaiting scientific review)
- About Antimicrobial Resistance — Government or standards body
- The misuse and overuse of antimicrobials are driving the development and spread of drug-resistant pathogens. (awaiting scientific review)
- Antimicrobial resistance (WHO fact sheet) — Government or standards body
Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.
Content status: published 1 October 2026.
- Scientific review: this version has not yet been signed off by a scientific reviewer.
- The Advanced explanation has not yet been reviewed for age suitability.