Evolution is often described as a change in inherited traits over generations. At the genetic level, that change can be understood more precisely: evolution is a change in the genetic composition of a population over time.
A key part of that process involves alleles, different versions of the same gene. Alleles arise through changes in DNA and can become more or less common in a population through natural selection, genetic drift, gene flow, and other evolutionary processes.
Understanding alleles provides a useful bridge between genetics and evolution. It explains not only why individuals differ, but also how those differences can accumulate and reshape populations across generations.
What is an allele?
An allele is a particular version of a gene. A gene is a segment of DNA involved in producing a functional product, such as a protein, or in regulating biological activity. The same gene can exist in multiple forms because DNA sequences can differ.
For example, a gene involved in producing a pigment might have one allele associated with one version of pigmentation and another allele associated with a different version. The important point is that alleles occupy corresponding locations, or loci, in the genome and represent alternative versions of the same genetic unit.
In organisms such as humans, which are typically diploid, an individual inherits two copies of most autosomal genes—one from each biological parent. Those copies may carry the same allele or different alleles. An individual with two identical alleles at a locus is homozygous; an individual with two different alleles is heterozygous.
The relationship between an allele and an observable trait is not always simple. Some alleles have dominant or recessive effects, but many traits depend on interactions among multiple genes and environmental factors. Consequently, an allele does not necessarily correspond to a single visible characteristic.
Where do new alleles come from?
New alleles ultimately arise through changes in DNA. A mutation is a change in the DNA sequence. Mutations can occur because of errors during DNA replication or because DNA is altered by other biological or environmental processes.
Most mutations do not produce a dramatic effect on an organism. Some have little or no detectable effect; others can alter biological function in ways that are harmful, beneficial, or context-dependent.
For an allele to contribute to evolutionary change across generations, the underlying genetic change generally must be heritable. In sexually reproducing organisms, mutations in cells that give rise to eggs or sperm can be passed to offspring. Mutations confined to ordinary body cells typically are not inherited by the next generation.
Mutation therefore supplies new genetic variation, but mutation alone does not determine which alleles become common. Evolutionary processes act on the variation that exists in populations.
How alleles relate to genetic variation
A population can contain multiple alleles at the same genetic locus. The proportion of individuals carrying particular alleles can vary from one population to another and can change over generations.
Suppose a population contains two alleles, A and a. If A occurs in 70 percent of the relevant gene copies and a occurs in 30 percent, those percentages describe the allele frequencies in the population.
Evolution can then be described as a change in these frequencies. If A rises from 70 percent to 85 percent over successive generations while a falls from 30 percent to 15 percent, the population has undergone evolutionary change at that locus.
This distinction matters because individuals do not evolve in the biological sense; populations do. An individual can grow, develop, learn, or change physiologically during its lifetime, but those changes are not what biologists mean by evolution. Evolution refers to inherited genetic change across generations within populations.
Natural selection can change allele frequencies
Natural selection occurs when individuals with certain heritable characteristics tend, under particular environmental conditions, to leave more surviving offspring than individuals with other characteristics.
If an allele contributes to a trait that improves reproductive success in a particular environment, individuals carrying that allele may, on average, contribute more copies of it to the next generation. Over time, the allele can become more common.
Consider a population of insects in which variation in body coloration is partly determined by different alleles. If predators more easily detect one coloration against the local background, insects with the less noticeable coloration may have greater survival and reproductive success. If the difference is heritable, the associated allele can increase in frequency.
Natural selection does not work because organisms consciously adapt themselves to their surroundings. Instead, existing heritable variation affects reproductive success, and allele frequencies change as a consequence.
Selection also depends on circumstances. An allele that is advantageous in one environment may be neutral or disadvantageous in another. Changes in climate, predators, food availability, disease, or other conditions can therefore alter which variants are favored.
Genetic drift changes alleles by chance
Not every change in allele frequency results from an advantage or disadvantage. Genetic drift is random change in allele frequencies caused by chance differences in which individuals reproduce and which alleles are passed on.
Drift is especially influential in small populations. Imagine a small population in which two alleles are present at roughly equal frequencies. By chance, individuals carrying one allele may produce more offspring in a particular generation. The next generation could therefore contain a noticeably different proportion of the two alleles, even if neither allele affects survival or reproduction.
Over many generations, genetic drift can cause alleles to become fixed, meaning one allele is the only version remaining at a locus in the population, or lost entirely.
Chance events can produce particularly strong effects when a population undergoes a sharp reduction in size. This is known as a population bottleneck. A different situation occurs when a small group establishes a new population; the resulting population may carry only a subset of the genetic variation present in the original population. This is called the founder effect.
Gene flow moves alleles between populations
Populations are not always genetically isolated. Individuals, or their reproductive cells, can move between populations and reproduce. This movement can introduce alleles into a population or remove them from one.
The transfer of genetic variants between populations is called gene flow.
For example, if individuals from one population regularly join another population and reproduce, alleles common in the first population can enter the second. Gene flow can make populations genetically more similar, although its evolutionary effects depend on which alleles are exchanged and on the circumstances of each population.
Migration is therefore not merely a geographic phenomenon. When migrants reproduce, it can also be a mechanism of genetic change.
Sexual reproduction reshuffles existing alleles
Sexual reproduction does not usually create new alleles directly, but it generates new combinations of existing ones.
During the formation of reproductive cells, meiosis separates chromosome copies and allows genetic material to be reshuffled. Processes such as independent assortment and crossing over produce different combinations of alleles. Fertilization then combines genetic material from two parents.
This reshuffling contributes substantially to genetic diversity within populations. It also means that natural selection can act on many different combinations of inherited variants.
Mutation introduces new variants; recombination and sexual reproduction rearrange existing variation; selection, drift, and gene flow can then alter how common those variants become.
Evolution does not always make an allele more common
An important misconception is that evolution necessarily produces organisms that are progressively better or more complex.
Evolution has no universal direction. An allele may become more common because it improves reproductive success in a particular environment, because of random genetic drift, or because it arrived through gene flow. An allele can also become less common or disappear.
Moreover, an allele that is beneficial in one setting may not be beneficial elsewhere. Evolution reflects the interaction between genetic variation and particular environmental and demographic conditions, not a march toward a predetermined ideal organism.
Alleles can have effects that depend on context
The evolutionary consequences of an allele can be difficult to predict because genes operate within biological systems.
An allele may affect several traits, a phenomenon known as pleiotropy. Different genes can also influence the same trait. The effect of one allele may depend on which alleles occur at other loci, a form of gene interaction.
Environmental conditions matter as well. Temperature, nutrition, pathogens, stress, and other factors can influence how genetic differences are expressed.
An allele can therefore have a different effect depending on an individual’s genetic background or environment. Evolutionary outcomes emerge from these interactions rather than from genes acting independently.
How selection, drift, mutation, and gene flow work together
The major evolutionary processes are easier to understand when considered together.
Mutation creates new genetic variants. Natural selection can increase or decrease the frequency of variants depending on their effects on reproductive success. Genetic drift changes frequencies through chance, particularly in small populations. Gene flow moves variants between populations. Recombination continually produces new combinations of existing alleles.
These processes can operate simultaneously. Selection might favor an allele while gene flow introduces competing variants. Drift might cause a neutral allele to become common by chance. Mutation might introduce a new allele that selection later acts upon.
The genetic composition of a population is therefore the outcome of interacting evolutionary forces rather than a single mechanism.
Alleles and adaptation are closely connected
Adaptation refers to an inherited characteristic that contributes to an organism’s ability to survive and reproduce in a particular environment, as well as the evolutionary process by which populations become better suited to those conditions.
Alleles can contribute to adaptation when genetic variants affect traits that influence reproductive success. Over generations, natural selection can increase the frequency of variants associated with advantageous traits.
But an adaptation is a population-level evolutionary outcome, not something an individual develops because it needs it. A population must already contain, or acquire through mutation and other genetic processes, variation on which selection can act.
This is why genetic variation is fundamental to evolution. Without heritable differences among individuals, natural selection could not produce evolutionary change in the usual way.
Why allele frequencies are central to evolutionary biology
Looking at allele frequencies gives scientists a precise way to track genetic evolution.
Instead of simply saying that a population has “changed,” researchers can ask which variants are present, how frequently they occur, and how those frequencies differ among populations or across generations. Changes in allele frequencies can reveal the effects of selection, drift, migration, population history, and other forces.
The concept also clarifies an important distinction between genotype and phenotype. A genotype describes an individual’s genetic makeup at one or more loci, while a phenotype refers to observable or measurable characteristics produced through the interaction of genetic factors and the environment.
Evolution acts on populations of genotypes, while natural selection is ultimately related to differences in survival and reproduction associated with phenotypic variation. Connecting those levels—from DNA variants to traits to reproductive success—is one of the central tasks of evolutionary genetics.
Alleles are therefore more than alternative versions of genes. They are the units of genetic variation whose frequencies can change through generations. By following those changes, evolutionary biology connects DNA-level differences with the large-scale patterns of adaptation, population divergence, and biological diversity seen in the natural world.
