In evolutionary biology, a population is a group of organisms of the same species that live in the same area and are capable of reproducing with one another. More importantly, evolution acts on populations rather than on individual organisms: a population changes genetically across generations as the frequencies of different inherited traits change.
This definition has an important implication. A population is not simply a collection of individuals that happen to look alike. Evolutionary biologists are interested in the shared gene pool—the collection of genetic variants present in a population—and how that gene pool changes over time.
Understanding what biologists mean by population helps explain natural selection, genetic drift, gene flow, mutation, adaptation, and the formation of new species.
A population is more than a group of individuals
An individual organism is born, lives, reproduces, and dies. Its genes generally do not change in ways that drive evolutionary change during its lifetime. Evolution is instead measured by comparing populations across generations.
Suppose a population of beetles contains genes that influence body color. If most beetles are light-colored in one generation but a much larger proportion carry genetic variants for dark coloration several generations later, the population’s genetic composition has changed. That is evolutionary change.
The individual beetles did not evolve into a new form during their lives. The population evolved because the relative frequencies of inherited genetic variants changed across generations.
This distinction is fundamental to evolutionary biology.
What makes something a population?
Biologists use the concept somewhat flexibly depending on the question being studied, but several characteristics are especially important.
Members belong to the same species
A population usually consists of organisms belonging to the same species. Members of a species share a common evolutionary history and, under the biological species concept, are generally capable of interbreeding and producing viable, fertile offspring.
There are exceptions and complications. Some closely related species can hybridize, and species boundaries are not always clear-cut. For evolutionary research, however, the key question is often how genetic material is distributed among organisms and groups rather than whether a boundary has an absolute definition.
Members occupy a particular geographic area
A population is typically associated with a particular location. A population of deer in one region, for example, may be distinguished from deer living elsewhere.
The boundaries do not have to be obvious. Animals can move, seeds can disperse, and pollen can travel between areas. As a result, populations can overlap or exchange individuals. Biologists often define population boundaries according to the biological question they are investigating.
Members can reproduce with one another
The individuals in a population are connected, at least potentially, through reproduction. This reproductive connection allows genes to pass between individuals and generations.
For evolutionary purposes, this matters because natural selection and other evolutionary processes alter the genetic composition of a connected group.
The population’s gene pool
The gene pool refers to all the genetic variants, or alleles, carried by members of a population.
An allele is a particular version of a gene or genetic sequence. A population can contain multiple alleles at a particular genetic location. For example, a gene involved in pigmentation might have several variants within a population.
Evolutionary biologists can track how common those variants are. If an allele becomes more or less common over generations, the population’s genetic composition has changed.
This is why evolutionary biology often describes evolution as a change in allele frequencies in a population over generations.
The idea also explains why genetic diversity matters. A population with many genetic variants may have more heritable variation on which evolutionary processes can act than a population with very little variation.
Why populations, rather than individuals, evolve
Individuals can develop, learn, acclimate, or change physiologically during their lifetimes, but those changes are not necessarily evolutionary.
For a characteristic to contribute directly to evolution, differences associated with it generally need to have a genetic basis and be capable of being inherited.
Consider a population of plants exposed to drought. Individual plants may respond physiologically by conserving water. That response does not, by itself, mean the population has evolved.
Now imagine that some plants carry inherited genetic variants that make them better able to survive drought. If those plants leave more offspring and their variants become more common in later generations, the population has undergone evolutionary change.
The important unit of change is therefore the population’s genetic composition, even though selection acts through differences among individual organisms.
How evolutionary forces change populations
Several major processes can alter the genetic composition of populations. They do not all operate in the same way.
Natural selection
Natural selection occurs when heritable differences among individuals affect their chances of surviving or reproducing.
If a particular inherited trait consistently gives its carriers an advantage in a particular environment, the genetic variants associated with that trait may become more common over generations.
Selection does not work toward a predetermined goal. A trait is favored only in relation to the conditions in which organisms live and reproduce.
Genetic drift
Genetic drift is random change in allele frequencies.
Chance can have a substantial effect, particularly in small populations. An allele can become common or disappear simply because its carriers happen, by chance, to leave more or fewer descendants.
Drift differs from natural selection because the change is not necessarily related to whether an allele improves an organism’s survival or reproduction.
Gene flow
Gene flow occurs when individuals or their reproductive cells move between populations and introduce genetic variants into another population.
Migration can therefore make populations genetically more similar. It can also introduce variants that were previously absent from a population.
The movement of pollen between plant populations and the migration of animals between neighboring populations are examples of processes that can produce gene flow.
Mutation
Mutation creates new genetic variants by altering DNA. Most mutations do not automatically provide an advantage, and many have little observable effect. Some can be harmful, while others can be beneficial in particular circumstances.
Mutation is important to evolution because it ultimately provides new genetic variation. Natural selection and genetic drift then affect the frequencies of variants already present or newly introduced.
Populations can be genetically different from one another
Two populations of the same species may live in different environments and have different genetic compositions.
For example, populations separated by geography may experience different climates, predators, diseases, or food sources. Natural selection may favor different traits in each environment. Genetic drift can also cause populations to diverge, particularly when they are small or isolated.
If individuals from the populations continue to exchange genes freely, however, gene flow can counteract some of that divergence.
Over long periods, genetic differences between populations can become substantial. In some cases, continued divergence contributes to speciation, the evolutionary process through which distinct species arise.
Population size matters
Population size affects how evolution proceeds.
Large populations tend to be less affected by random fluctuations in allele frequencies because chance events have a smaller proportional effect. Small populations are more vulnerable to genetic drift, which can rapidly change allele frequencies and reduce genetic variation.
A population can also experience a population bottleneck when its size is sharply reduced by an event such as a severe environmental disturbance. The surviving individuals carry only a portion of the genetic variation that existed before the reduction, so the population’s genetic composition may change substantially.
A related phenomenon, the founder effect, occurs when a new population is established by a small number of individuals. The genetic composition of the founders may differ from that of the larger source population simply because of chance.
Population boundaries are not always simple
In nature, populations rarely behave like neatly separated boxes.
A species may occupy a large geographic range containing many partially connected populations. Individuals may move between neighboring groups, producing varying amounts of gene flow. Some organisms also have highly mobile life stages, making geographic boundaries particularly difficult to define.
Biologists therefore may use different population definitions for different purposes. A conservation biologist might study a population isolated by habitat fragmentation. A geneticist might define populations according to measurable patterns of genetic similarity. An ecologist might focus on organisms occupying a particular habitat.
These approaches are not necessarily contradictory. They emphasize different aspects of the same underlying biological reality.
Population versus community and species
Several biological terms describe groups of organisms, but they refer to different levels of organization.
| Term | Meaning |
|---|---|
| Individual | One organism |
| Population | Members of the same species occupying a particular area and connected by reproduction |
| Community | Populations of different species living and interacting in an area |
| Ecosystem | A community together with its physical environment |
| Species | A broader biological category encompassing populations that share an evolutionary lineage and, depending on the species concept, reproductive or genetic relationships |
The distinction between a population and a species is especially important. A species can contain many populations. Those populations may differ genetically while remaining part of the same species.
Why the population concept is central to evolution
The population is where individual genetic differences become changes in a species over time.
Individuals provide the variation. Reproduction passes genetic variants to subsequent generations. Environmental conditions can influence which individuals leave more descendants. Random events can alter which variants persist. Migration can move variants between populations, and mutation can create new ones.
Taken together, these processes change the genetic composition of populations. When such changes accumulate over many generations, they can produce adaptations, substantial divergence among populations, and eventually new species.
In that sense, a population is the bridge between individual organisms and large-scale evolutionary change: evolution is expressed through changes in populations, while those changes arise from processes acting on the organisms and genes that make up those populations.
