Natural selection is the process by which inherited traits that improve an organism’s ability to survive and reproduce tend to become more common in a population over generations. It is one of the central mechanisms of evolution, but natural selection does not happen simply because organisms differ or because some organisms survive better than others.
For natural selection to produce evolutionary change, four conditions must occur together:
- Variation: Individuals in a population differ in their traits.
- Heritability: Some of those differences can be passed from parents to offspring.
- Differential survival or reproduction: Individuals with certain traits leave more surviving offspring than others.
- A connection between the trait and reproductive success: The trait differences must contribute to those differences in survival or reproduction.
These conditions are closely related, but each plays a distinct role. If one is missing, natural selection cannot cause the corresponding inherited trait to become more common through selection.
1. Variation must exist in the population
Natural selection requires differences among individuals. A population cannot be shaped by selection if every individual has exactly the same relevant traits.
Variation can involve physical characteristics, physiological processes, behavior, or other traits. For example, members of a population might differ in body size, coloration, tolerance to temperature, resistance to a disease, or the timing of reproduction.
The existence of variation alone, however, does not mean natural selection is occurring. Differences can arise for many reasons and may have no effect on reproductive success. A population might contain substantial variation in a trait that makes no difference in its environment.
Variation also does not have to be created by natural selection itself. Genetic variation can arise through processes such as mutation and genetic recombination. Once variation exists, selection can favor some variants over others when environmental conditions make those differences important.
2. Some of the variation must be heritable
For natural selection to produce evolutionary change, at least some of the relevant differences must be heritable, meaning they can be transmitted from parents to offspring.
This condition is crucial because natural selection operates across generations. Suppose one group of organisms happens to be larger than another, but their size difference results entirely from differences in nutrition during development and is not inherited. If larger individuals leave more offspring, their offspring will not necessarily be larger as a result. The population therefore would not become genetically larger simply through that episode of selection.
Heritability does not mean that a trait is determined entirely by genes. Many traits are influenced by both genes and environmental conditions. What matters for natural selection is whether some of the variation associated with differences in reproductive success can be passed to the next generation.
This is also why natural selection should be distinguished from changes that occur within an individual’s lifetime. An organism can change because it learns, grows, becomes acclimated to its surroundings, or experiences environmental effects. Those changes are not automatically evolutionary changes. For selection to alter a population over generations, inherited differences must be involved.
3. Individuals must differ in survival or reproduction
Natural selection requires differential reproductive success: some individuals contribute more offspring to the next generation than others.
This can happen because individuals differ in their ability to survive long enough to reproduce, but survival itself is not the ultimate requirement. An organism that survives for a long time but produces no offspring does not contribute its genes to future generations.
Differences in reproductive success can result from many aspects of an organism’s life. One individual may be better able to obtain food, avoid predators, resist infection, tolerate environmental stress, attract mates, produce viable offspring, or care for young. Another may reproduce less successfully or fail to reproduce at all.
The relevant measure is therefore often called fitness. In evolutionary biology, fitness does not simply mean being physically strong or healthy. It refers to an organism’s relative contribution of viable offspring to subsequent generations.
Fitness is also context-dependent. A trait that increases reproductive success in one environment may be neutral or harmful in another. Thick fur, for example, can be advantageous in a cold environment but costly in a hot one.
4. The inherited variation must affect reproductive success
The final condition connects the first three. The trait differences must actually be related to differences in survival or reproduction.
Imagine a population containing individuals with different eye colors. If eye color has no effect on their chances of surviving or reproducing, natural selection has no reason to favor one color over another. The variation and heritability may be present, but selection will not change the frequency of the colors because of that trait.
Now suppose a particular inherited coloration makes some individuals harder for predators to detect. If those individuals consequently survive and reproduce more successfully, their offspring are more likely to inherit the advantageous coloration. Over generations, the trait can become more common.
The important point is that selection acts on differences in reproductive success, not on traits simply because they are different. A trait becomes subject to natural selection when variation in that trait is associated with a consistent difference in reproductive success under particular environmental conditions.
How the four conditions work together
Consider a hypothetical population of insects living on plants with different backgrounds. Some insects are lighter and others darker.
There is variation because the insects differ in coloration. Suppose coloration has a genetic component, providing heritability. If birds more easily detect the insects that contrast with the plants, coloration can influence survival and reproduction. The better-camouflaged insects may survive more often and leave more offspring, creating differential reproductive success associated with the inherited trait.
If these conditions persist over generations, the frequency of the better-camouflaged form can increase in the population.
The insects themselves do not deliberately change their coloration because they need better camouflage. Instead, individuals already differ, some of those differences are inherited, and the environment affects which individuals leave more descendants. The resulting change in the population is natural selection.
Natural selection acts on populations across generations
A common misunderstanding is that natural selection causes individual organisms to evolve. An individual organism does not change its inherited characteristics because the environment demands it. Instead, individuals with different inherited traits can have different reproductive outcomes.
Populations evolve; individuals are selected.
For example, if a population contains bacteria with different levels of resistance to an antibiotic, treatment can kill susceptible bacteria while resistant bacteria survive and reproduce. The surviving bacteria can therefore make up a larger proportion of the population in later generations. The population has changed in its genetic composition.
This distinction also explains why natural selection is fundamentally a generational process. A single organism’s survival may contribute to selection, but evolutionary change occurs when differences in reproductive success accumulate across generations.
Natural selection does not require perfection
Natural selection does not necessarily produce organisms that are perfectly adapted to their environments. It favors heritable variants that, under particular conditions, contribute more successfully to reproduction than competing variants.
Environmental conditions can change, and a trait that was advantageous in one setting may become less useful in another. In addition, organisms face trade-offs. A trait that improves one aspect of survival or reproduction may carry a cost elsewhere.
Natural selection also works with existing variation. It does not plan ahead or generate a trait because an organism needs it. Mutations and other genetic processes provide sources of variation, while selection changes the relative frequency of variants according to their effects on reproductive success.
A simple way to recognize natural selection
When evaluating whether a situation involves natural selection, ask four questions:
Are individuals different? If there is no relevant variation, selection cannot favor one form over another.
Are some of those differences inherited? Without heritable variation, differences between generations cannot be reliably transmitted through reproduction.
Do individuals with different traits leave different numbers of viable offspring? If reproductive success is the same, selection is not favoring one variant.
Is the difference in reproductive success connected to the inherited trait? If the trait has no bearing on fitness, its frequency will not increase through selection for that trait.
When the answer to all four questions is yes, the basic requirements for natural selection are present. Over successive generations, the inherited traits associated with greater reproductive success can become more common, producing evolutionary change in the population.
