Natural selection is the evolutionary process in which heritable traits that improve reproductive success tend to become more common in a population over generations. It occurs because individuals vary, some of that variation is inherited, and individuals with different traits do not all leave the same number of surviving offspring.
Natural selection is one of the main mechanisms of evolution. It explains how populations become adapted to their environments and helps account for the enormous diversity of life on Earth. Charles Darwin and Alfred Russel Wallace independently developed the idea in the nineteenth century, although modern genetics has greatly expanded our understanding of how selection works.
Key Takeaways: Natural Selection
- Natural selection is a mechanism of evolution in which heritable traits affecting reproductive success change in frequency over generations.
- Natural selection requires variation, inheritance, and differences in reproductive success.
- Populations evolve; individual organisms do not evolve by natural selection.
- Natural selection acts on existing phenotypes, while evolutionary change occurs as the genetic composition of a population changes.
- Selection is not random, although mutations and other processes that produce genetic variation are not directed toward what an organism “needs.”
- The three classic patterns are directional, stabilizing, and disruptive selection.
- Natural selection does not work toward a predetermined goal or produce perfect organisms.
Why Natural Selection Is Important
Natural selection provides a mechanism explaining how populations change in response to their environments. Before the nineteenth century, scientists had proposed that species could change, but there was no widely accepted mechanism explaining how adaptive changes arose.
Natural selection connects many areas of biology. It helps explain:
- Adaptations, such as camouflage, drought tolerance, and resistance to disease
- Changes in populations over time
- The origin and diversification of species
- Antibiotic resistance in bacteria
- Pesticide and herbicide resistance
- Interactions between predators and prey, hosts and parasites, and plants and pollinators
- Why some inherited diseases remain in populations
- The relationship between organisms and their environments
Natural selection is therefore central to evolutionary biology, ecology, genetics, medicine, agriculture, and conservation biology.
History of Natural Selection
Ideas about species changing over time existed long before Darwin.
In 1809, French naturalist Jean-Baptiste Lamarck published Philosophie Zoologique. Lamarck argued that organisms changed in response to their environments and that characteristics acquired during an organism’s lifetime could be inherited. His proposed mechanism was largely incorrect, but his work was important because it treated species as capable of changing rather than as permanently fixed.
British geologist Charles Lyell published the first volume of Principles of Geology in 1830. Lyell argued that geological features resulted largely from processes operating over immense spans of time. His work helped establish the concept of an ancient, changing Earth, providing enough time for gradual biological evolution.
From 1831 to 1836, Charles Darwin traveled aboard HMS Beagle. His observations of organisms, fossils, geology, and geographic distributions helped shape his ideas about evolution. Darwin’s observations of Galápagos mockingbirds were particularly important to his early thinking about the mutability of species. The famous Darwin’s finches became significant evidence as their relationships were studied after the voyage.
Darwin was also influenced by economist Thomas Robert Malthus, whose An Essay on the Principle of Population, first published in 1798, argued that populations have the potential to increase faster than their resources. Darwin recognized that this meant more organisms are produced than can survive and reproduce.
By 1838, Darwin had developed the basic idea of natural selection, but he did not immediately publish it.
Meanwhile, British naturalist Alfred Russel Wallace independently arrived at a remarkably similar explanation while studying organisms in the Malay Archipelago. In 1858, Wallace sent Darwin an essay describing his idea. Papers by Darwin and Wallace were jointly presented to the Linnean Society of London on July 1, 1858.
Darwin published On the Origin of Species in 1859, presenting extensive evidence for evolution and natural selection. Wallace continued making major contributions to evolutionary biology and biogeography, although Darwin’s name eventually became more strongly associated with natural selection.
One problem remained: Darwin did not know how inheritance worked.
Gregor Mendel reported his experiments with pea plants in 1865, establishing principles that later became the foundation of genetics. His work received little attention until its rediscovery around 1900.
During the early twentieth century, researchers united Mendelian genetics with natural selection. Work by scientists including Ronald Fisher, J. B. S. Haldane, and Sewall Wright established population genetics. During the 1930s and 1940s, their work and that of researchers such as Theodosius Dobzhansky, Ernst Mayr, Julian Huxley, George Gaylord Simpson, and G. Ledyard Stebbins contributed to the modern evolutionary synthesis, which connected genetics, natural selection, paleontology, systematics, and other areas of biology.
Natural Selection Definition
Natural selection is differential survival and reproduction resulting from differences among individuals in heritable traits.
In simpler terms, organisms in a population differ from one another. When some inherited differences cause their bearers to leave more offspring than others, those traits tend to become more common in later generations.
Natural selection acts directly on the phenotype, which consists of observable characteristics influenced by genes and environment. When phenotypic differences have a heritable genetic basis, selection can change allele frequencies and other aspects of the genetic composition of the population.
Requirements for Natural Selection
Natural selection requires several conditions.
1. Variation
Individuals within a population differ. They may vary in size, coloration, physiology, behavior, disease resistance, reproductive timing, or thousands of other characteristics.
2. Heritability
At least some of these differences must be inherited. A characteristic caused entirely by environmental conditions cannot evolve by natural selection unless differences in the response itself have a heritable basis.
3. Differences in Reproductive Success
Individuals do not contribute equally to the next generation. Some survive longer, attract more mates, produce more offspring, or produce offspring that are themselves more likely to survive and reproduce.
4. Multiple Generations
When these differences persist over generations, advantageous heritable traits can increase in frequency within the population.
How Natural Selection Works
Consider a hypothetical population of insects that varies in color from pale brown to dark brown.
Suppose the insects live on dark tree bark and birds hunt them by sight. Dark insects are somewhat harder for birds to detect. If body color is heritable, darker insects may survive predation more often and produce more offspring.
The next generation still contains variation, but genes associated with darker coloration may now be slightly more common. If the same selective pressure continues, the population may become progressively darker over many generations.
The important point is that individual insects did not become darker because they needed camouflage. Darker variants already existed. The environment affected which variants reproduced most successfully.
Natural selection therefore sorts existing heritable variation rather than deliberately creating the traits organisms require.
Where Does Variation Come From?
Natural selection cannot operate without variation.
New genetic variation ultimately arises through mutation, which changes DNA. In sexually reproducing organisms, crossing over, independent assortment, and random fertilization continually produce new combinations of existing alleles.
Gene flow, the movement of alleles between populations, can also introduce genetic variation into a population.
Mutations do not occur because an organism needs a particular adaptation. A mutation may be beneficial, harmful, or effectively neutral depending on its effects and the environment.
Natural selection then influences the reproductive success associated with this variation.
Fitness and “Survival of the Fittest”
In evolutionary biology, fitness means reproductive success relative to other members of a population.
Fitness does not necessarily mean being the biggest, strongest, fastest, or longest-lived organism.
A physically strong animal that produces no offspring has low evolutionary fitness. A smaller animal that produces many surviving, reproducing offspring can have much higher fitness.
The phrase “survival of the fittest” can therefore be misleading. Survival matters only insofar as it contributes to reproduction. In addition, an organism’s fitness depends on its environment. A trait that is advantageous under one set of conditions may be disadvantageous under another.
Types of Natural Selection
The three classic patterns of natural selection describe how selection affects the distribution of a trait within a population.
| Type | What Is Favored | Typical Result | Example |
|---|---|---|---|
| Directional selection | One extreme phenotype | Population average shifts | Antibiotic resistance |
| Stabilizing selection | Intermediate phenotype | Variation around the average decreases | Human birth weight |
| Disruptive selection | Both extreme phenotypes | Intermediate forms become less common | Different beak sizes specialized for different foods |
Directional Selection
Directional selection favors individuals at one end of a phenotypic range.
Suppose a population of birds varies in beak depth. During a drought, mostly large, hard seeds remain available. Birds with deeper, stronger beaks may obtain more food and leave more offspring. The population’s average beak depth can shift toward larger values.
Evolution of resistance to antibiotics, pesticides, and herbicides also frequently involves directional selection.
Stabilizing Selection
Stabilizing selection favors intermediate phenotypes over either extreme.
Human birth weight provides a classic example. Historically, extremely small infants faced increased risks associated with underdevelopment, while extremely large infants faced greater risks during delivery. Intermediate birth weights tended to have higher survival.
Stabilizing selection reduces variation around an optimum without necessarily changing the population average very much.
Disruptive Selection
Disruptive selection, also called diversifying selection, favors individuals at both extremes over intermediate individuals.
For example, imagine birds that feed primarily on either small, soft seeds or large, hard seeds. Small beaks may efficiently handle small seeds and large beaks may crack large seeds, while intermediate beaks perform poorly at both tasks.
Disruptive selection can increase variation within a population and, under some circumstances, contribute to population divergence.
Other Forms of Selection
Not every biologically important form of selection fits neatly into the three classic patterns.
Sexual Selection
Sexual selection results from differences in success at obtaining mates or achieving fertilization. Darwin discussed it extensively in The Descent of Man, and Selection in Relation to Sex in 1871.
Sexual selection can involve competition between individuals for mates or mate choice. Peacock tails, deer antlers, elaborate courtship displays, and many differences between males and females have been influenced by sexual selection.
A sexually selected trait can increase reproductive success even if it carries a survival cost.
Balancing Selection
Balancing selection maintains multiple alleles or phenotypes within a population rather than consistently eliminating all but one.
One mechanism is heterozygote advantage. The classic example involves the sickle-cell allele and malaria. In regions where malaria has historically been common, individuals heterozygous for the sickle-cell allele have increased resistance to severe malaria compared with individuals lacking the allele, helping maintain the allele in the population.
Another mechanism is frequency-dependent selection, in which the fitness of a phenotype depends on how common or rare it is.
Examples of Natural Selection
Antibiotic Resistance
A bacterial population may contain genetic variants that differ in susceptibility to an antibiotic. Treatment kills susceptible bacteria more effectively, while resistant bacteria survive and reproduce.
The antibiotic does not cause bacteria to develop resistance because they “try” to survive. Instead, antibiotic exposure creates strong selection favoring resistance that is already present or subsequently arises through genetic change.
Darwin’s Finches
Long-term studies of Galápagos finches have documented changes in beak characteristics associated with environmental conditions and food availability. Changes can occur measurably over relatively short periods when selection is strong.
Peppered Moths
The peppered moth (Biston betularia) is a famous example involving camouflage and predation. Changes in pollution altered tree surfaces in industrial Britain, changing the relative camouflage and survival of light and dark moths. Later reductions in pollution changed the selective environment again.
Pesticide Resistance
Repeated pesticide use can favor insects carrying alleles that confer resistance. Over generations, resistant individuals become increasingly common, making the pesticide less effective.
Lactase Persistence
Most mammals reduce production of the enzyme lactase after weaning. In several human populations with long histories of dairying, genetic variants that maintain lactase production into adulthood increased in frequency. This is an example of natural selection interacting with human culture.
Natural Selection vs. Evolution
Natural selection and evolution are related, but they are not synonyms.
Evolution is a change in the heritable characteristics of a population across generations. At the genetic level, this includes changes in allele frequencies.
Natural selection is one mechanism capable of producing evolutionary change.
Evolution can also occur through:
- Genetic drift: Random changes in allele frequencies, especially important in small populations
- Gene flow: Movement of alleles between populations
- Mutation: Creation of new genetic variants
Natural selection differs from genetic drift because selection systematically relates differences in phenotype to differences in reproductive success, while genetic drift results from chance sampling.
Not every evolutionary change is an adaptation, and not every feature of an organism exists because natural selection favored it.
Adaptation and Natural Selection
An adaptation is a heritable characteristic that evolved because it increased fitness in a particular environment.
Natural selection can produce adaptations because beneficial heritable traits tend to increase in frequency. But adaptations are not perfect solutions.
Evolution works with available variation and with structures inherited from ancestors. Traits may involve trade-offs, genes may affect several characteristics at once, and environments continually change.
As a result, organisms are generally well enough adapted to reproduce, not perfectly engineered for every situation.
Natural Selection vs. Artificial Selection
Artificial selection occurs when humans choose which organisms reproduce based on desired traits. Natural selection occurs when differences in reproductive success arise from interactions between organisms and their environments.
| Natural Selection | Artificial Selection |
|---|---|
| Environment determines which traits affect fitness | Humans choose desired traits |
| Occurs in natural populations | Common in domesticated organisms |
| Does not pursue a predetermined goal | Usually has a human-defined goal |
| Produces adaptation to environmental conditions | Produces traits valued by breeders |
| Examples include camouflage and antibiotic resistance | Examples include dog breeds and crop varieties |
Darwin used artificial selection as an analogy for natural selection. His studies of domesticated pigeons demonstrated how substantial changes could accumulate when particular heritable traits repeatedly influenced which organisms reproduced.
Natural Selection vs. Lamarckism
Natural selection and Lamarckism both attempted to explain how organisms change over time, but they propose very different mechanisms.
| Natural Selection | Lamarckism |
|---|---|
| Heritable variation exists among individuals | Organisms change through use or disuse |
| Selection acts on existing differences | Environmental need directly produces useful change |
| Individuals differ in reproductive success | Individuals acquire characteristics during life |
| Populations change across generations | Acquired characteristics are passed to offspring |
| Supported by modern genetics and evolutionary biology | Classical mechanism is not supported as a general explanation of evolution |
A traditional example illustrates the difference.
A Lamarckian explanation of giraffe necks proposes that ancestral giraffes repeatedly stretched to reach leaves. Their necks became longer during life, and they passed this acquired characteristic to their offspring.
A natural-selection explanation starts with inherited variation in neck length and related traits. Individuals whose characteristics increased reproductive success left more descendants. Across many generations, alleles contributing to advantageous traits increased in frequency.
Modern discoveries in epigenetics sometimes cause confusion about Lamarckism. Some environmentally influenced epigenetic states can persist through cell divisions and, in certain cases, across generations. However, this does not restore classical Lamarckism as a general replacement for evolution by mutation, inheritance, selection, drift, and gene flow.
What Natural Selection Is and Is Not
Natural selection is:
- A mechanism of evolution
- Based on differences in reproductive success
- Dependent on heritable variation
- Influenced by environmental conditions
- Capable of producing adaptation
- A process acting across generations
However, natural selection is not:
- A conscious process
- A force that gives organisms what they need
- Entirely random
- The same thing as evolution
- Necessarily a process in which the physically strongest survive
- A process through which individual organisms evolve
- A guarantee of increasing complexity
- A mechanism that inevitably produces perfection
Natural Selection and Speciation
Natural selection can contribute to the formation of new species, although selection alone does not automatically cause speciation.
Suppose populations of the same species occupy different environments. Different traits may be advantageous in each environment, causing the populations to diverge genetically and phenotypically.
If divergence eventually contributes to reproductive isolation, so that members of the populations no longer exchange genes effectively, they may become separate species.
Natural selection is therefore an important mechanism behind adaptive divergence, but geographic isolation, genetic drift, sexual selection, and other processes can also contribute to speciation.
Can Natural Selection Be Observed?
Natural selection is not restricted to changes occurring over millions of years. Scientists observe it directly in organisms with short generation times and can measure it across generations in wild populations.
Antibiotic resistance, pesticide resistance, changes in viral populations, and measurable changes in animal and plant traits provide opportunities to study selection as it occurs.
Researchers can test for natural selection by determining whether:
- Individuals vary in a trait.
- The trait is heritable.
- Different trait values correlate with differences in reproductive success.
- The distribution or genetic basis of the trait changes across generations.
Natural selection therefore produces predictions that can be tested using experiments, field observations, genetics, fossils, and comparative biology.
Common Misconceptions About Natural Selection
“Organisms evolve because they need to.”
Need does not direct evolution. Variation exists first, and environmental conditions affect the reproductive success of different variants.
“Natural selection gives organisms useful mutations.”
Mutations arise without regard to whether they would be useful. Natural selection can subsequently increase the frequency of beneficial variants.
“Only the strongest survive.”
Evolutionary fitness refers primarily to reproductive success, not physical strength. Cooperation, camouflage, fertility, disease resistance, parental care, and many other characteristics can influence fitness.
“Individuals evolve.”
An individual can grow, develop, learn, and change physiologically, but evolutionary change occurs in populations across generations.
“Natural selection is completely random.”
The origins and inheritance of variation include stochastic processes, but natural selection itself is nonrandom with respect to fitness: heritable differences consistently associated with greater reproductive success tend to increase.
“Evolution always produces more complex organisms.”
There is no universal evolutionary trend toward greater complexity. Natural selection favors traits that increase reproductive success under particular conditions. Simplification can be advantageous.
“Every characteristic is an adaptation.”
Some traits result from genetic drift, developmental constraints, correlations with other traits, or evolutionary history rather than direct selection for their current form.
“Natural selection produces perfect organisms.”
Selection is constrained by existing variation, evolutionary history, trade-offs, and changing environments. Evolution produces workable adaptations, not perfection.
“Natural selection is just survival of the fittest.”
Survival is only part of fitness. Ultimately, selection depends on differential reproductive success.
Frequently Asked Questions
What is natural selection in simple terms?
Natural selection is the process by which inherited traits that help organisms leave more offspring tend to become more common in a population over generations.
Who discovered natural selection?
Charles Darwin and Alfred Russel Wallace independently developed the theory of evolution by natural selection. Their ideas were jointly presented in 1858. Darwin published On the Origin of Species in 1859.
What are the three main types of natural selection?
The three classic patterns are directional selection, stabilizing selection, and disruptive selection.
What are the four basic steps of natural selection?
A useful simplified sequence is variation, inheritance, differential reproductive success, and change across generations.
Is natural selection random?
Not entirely. Mutations arise without regard to what would benefit an organism, and chance affects reproduction and survival. But natural selection is nonrandom in the sense that heritable traits associated with greater reproductive success tend to increase in frequency.
Does natural selection act on individuals or populations?
Selection acts through differences among individuals, but the resulting evolutionary change occurs in populations across generations.
Does natural selection create new genes?
No. Mutation and other genetic processes generate variation. Natural selection changes the relative reproductive success of genetic variants.
Is natural selection the same as evolution?
No. Natural selection is one mechanism of evolution. Mutation, genetic drift, and gene flow also contribute to evolutionary change.
Can natural selection occur without evolution?
Selection may occur without producing a lasting evolutionary response if the relevant differences are not heritable or if other evolutionary forces counteract its effects.
Does natural selection still occur in humans?
Yes. Humans remain genetically variable populations in changing biological and cultural environments, so natural selection continues to operate.
How fast does natural selection occur?
There is no fixed rate. Detectable evolutionary changes can occur within a few generations when selection is strong, while other changes accumulate over thousands or millions of years.
Glossary
Adaptation: A heritable characteristic that evolved because it increased reproductive success in a particular environment.
Allele: One of two or more versions of a gene or genetic locus.
Artificial selection: Human-directed breeding that favors chosen heritable characteristics.
Balancing selection: Selection that maintains multiple genetic variants within a population.
Directional selection: Selection favoring one extreme of a phenotypic distribution.
Disruptive selection: Selection favoring both extremes of a phenotypic distribution over intermediate forms.
Evolution: Change in the heritable characteristics of a population across generations.
Fitness: An individual’s reproductive success relative to others in a population.
Gene flow: Movement of genetic variants between populations.
Genetic drift: Random change in allele frequencies resulting from chance.
Heritable: Capable of being transmitted genetically from parents to offspring.
Mutation: A change in genetic material that is an ultimate source of new genetic variation.
Natural selection: Differential survival and reproduction associated with differences in heritable traits.
Phenotype: The observable characteristics of an organism resulting from interactions between its genotype and environment.
Population: A group of individuals of the same species living and potentially reproducing within an area.
Sexual selection: Selection resulting from differences in success at obtaining mates or achieving fertilization.
Stabilizing selection: Selection favoring intermediate phenotypes over extremes.
Variation: Differences among individuals within a population.
Summary
Natural selection is a mechanism of evolution based on heritable variation and differences in reproductive success. Individuals with characteristics that improve reproductive success under particular environmental conditions tend to contribute more genes to future generations. As a result, populations change over time.
Darwin and Wallace independently developed the concept of natural selection in the nineteenth century, while later discoveries in genetics supplied the mechanism of inheritance that their original theories lacked. Modern evolutionary biology recognizes natural selection as one of several mechanisms of evolution, alongside mutation, genetic drift, and gene flow.
Natural selection does not give organisms the traits they need, make individuals evolve, or guide life toward perfection. Instead, it acts on heritable differences that already occur within populations. Over generations, this deceptively simple process can produce adaptation, contribute to speciation, and help explain much of the diversity of life.
References and Further Reading
- Begon, Michael; Townsend, Colin R.; Harper, John L. (1996). Ecology: Individuals, Populations and Communities (3rd ed.). Oxford; Cambridge, MA: Blackwell Science. ISBN 978-0-632-03801-5.
- Bowler, Peter J. (2003). Evolution: The History of an Idea (3rd ed.). Berkeley, CA: University of California Press. ISBN 978-0-520-23693-6.
- Darwin, Charles (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1st ed.). London: John Murray.
- Henry, Devin (2006). “Aristotle on the Mechanism of Inheritance”. Journal of the History of Biology. 39 (3): 425–455. doi:10.1007/s10739-005-3058-y
- Mills, Susan K.; Beatty, John H. (1979). “The Propensity Interpretation of Fitness”. Philosophy of Science. 46 (2): 263–286. doi:10.1086/288865