Evolution is the process by which populations of organisms change genetically across generations. It explains how populations become adapted to their environments, why closely related species can differ, and how the enormous diversity of life arose.
At its core, evolution depends on changes in allele frequencies—the proportions of different versions of genes within a population. Several biological processes can change those frequencies. Five major mechanisms are especially important: mutation, natural selection, genetic drift, gene flow, and nonrandom mating.
These mechanisms do not all work in the same way. Mutation introduces genetic variation; natural selection tends to favor variants associated with greater reproductive success; genetic drift changes populations through chance; gene flow moves genetic variants between populations; and nonrandom mating changes how individuals pair and how genetic combinations occur. Understanding those differences makes evolution much easier to recognize and explain.
Mutation creates new genetic variation
A mutation is a change in an organism’s DNA sequence. Mutations can arise from errors during DNA replication, from damage to DNA, or from other cellular processes. They can affect a single DNA base, a larger segment of DNA, or even the structure or number of chromosomes.
Mutation is the ultimate source of new genetic variants, or alleles, in a population. For example, a DNA change might produce a new version of a gene involved in pigmentation, metabolism, or resistance to a particular disease.
Most mutations do not automatically make an organism better or worse. Their effects depend on the gene involved, the environment, and the particular change in DNA. A mutation may have a harmful effect, a beneficial effect, or little detectable effect on an organism’s traits.
Importantly, a mutation in an individual’s body cells generally is not passed to offspring. For a mutation to contribute directly to evolutionary change across generations in animals, it typically must occur in a cell lineage that contributes to reproduction, such as a germ cell or its precursors.
Mutation by itself does not necessarily cause adaptation. It creates genetic possibilities; other evolutionary processes determine what happens to those variants in a population.
Natural selection favors inherited differences that affect reproduction
Natural selection occurs when individuals with different heritable traits leave different numbers of surviving offspring. If a trait consistently improves reproductive success in a particular environment, genetic variants contributing to that trait can become more common over generations.
Consider a population of insects that varies in coloration. If birds more easily detect one color against the local background, insects with better camouflage may survive to reproduce more often. If the color difference has a genetic basis, the associated alleles can increase in frequency over generations.
Natural selection does not work toward a predetermined goal. An organism does not evolve a useful trait because it needs one. Instead, existing heritable variation is filtered through differences in survival and reproduction.
The environment is therefore central to selection. A trait that is advantageous under one set of conditions may provide little benefit—or even become disadvantageous—when conditions change. Selection can produce adaptations, but adaptation is a population-level outcome of generations of differential reproduction, not an intentional process.
Natural selection can also take different forms. Directional selection favors one end of a range of traits, while stabilizing selection favors intermediate forms. Disruptive selection can favor individuals at both extremes over intermediate forms. Sexual selection, in which traits affect success in obtaining mates, is another important form of selection.
Genetic drift changes populations through chance
Genetic drift is evolutionary change caused by random differences in which individuals survive and reproduce. Unlike natural selection, drift does not favor alleles because they are beneficial.
Imagine a population in which two alleles are equally capable of producing healthy organisms. By chance, individuals carrying one allele might leave more offspring in a particular generation. The allele could then become more common even though it provided no reproductive advantage.
Drift tends to have stronger effects in small populations, where random events can produce relatively large changes in allele frequencies.
Two familiar examples illustrate this process. A bottleneck occurs when a population experiences a sharp reduction in size, leaving a surviving group whose genetic composition may differ substantially from that of the original population. A founder effect occurs when a small group establishes a new population, carrying with it only a portion of the genetic variation found in the source population.
Genetic drift can reduce genetic variation within populations and can cause populations to diverge from one another. It can even cause an allele to become fixed, meaning that it is the only allele of that gene present in the population, or to disappear entirely.
Gene flow moves alleles between populations
Gene flow is the movement of genetic material between populations. It can occur when organisms move and reproduce in another population, or when reproductive cells, such as pollen, move between populations.
For example, if individuals from one population migrate into another and successfully reproduce, alleles from the first population can enter the second. Over time, repeated gene flow can change allele frequencies in both populations.
Gene flow often makes populations more genetically similar because it introduces alleles from one population into another. It can also introduce useful genetic variation into a population that previously lacked certain alleles.
Its evolutionary effects depend on circumstances. If populations are exposed to different environments, substantial gene flow can sometimes counteract local adaptation by continually introducing alleles favored elsewhere. Conversely, gene flow can provide genetic variation that selection can act upon.
Gene flow is therefore important not only within species but also in understanding how geographically separated populations remain connected genetically.
Nonrandom mating changes how genetic combinations occur
In many basic discussions of evolution, the fifth mechanism is described as nonrandom mating. Strictly speaking, nonrandom mating does not necessarily change allele frequencies directly. Instead, it changes the frequencies of genotypes—the particular combinations of alleles individuals carry—and can therefore influence how evolutionary processes operate.
Under random mating, individuals pair without regard to particular heritable traits. Nonrandom mating occurs when mate choice is influenced by characteristics such as size, coloration, behavior, or geographic location.
Assortative mating occurs when individuals with similar traits are more likely to mate. Disassortative mating favors mating between individuals with different traits. Sexual selection can produce strong nonrandom mating when individuals preferentially choose certain mates.
Nonrandom mating can alter the distribution of homozygous and heterozygous individuals in a population. For example, mating between similar individuals can increase the frequency of homozygous genotypes without necessarily changing the overall allele frequencies.
This distinction matters because evolutionary change is often discussed in terms of allele frequencies, while natural populations involve several layers of genetic organization. Nonrandom mating can reshape genotype frequencies and influence the opportunity for selection to act, even when it is not itself directly changing allele frequencies.
How the five mechanisms differ
The mechanisms are easiest to understand when their roles are separated:
| Mechanism | Basic effect | Is chance important? | Typical evolutionary consequence |
|---|---|---|---|
| Mutation | Creates new alleles | Yes, in the occurrence of mutations | Introduces new genetic variation |
| Natural selection | Differential reproductive success | Not in the sense of random genetic change | Can increase advantageous heritable traits |
| Genetic drift | Random changes in allele frequencies | Yes | Can cause alleles to become common, rare, fixed, or lost |
| Gene flow | Moves alleles between populations | Depends on movement and reproduction | Often makes populations more genetically similar |
| Nonrandom mating | Changes patterns of mating and genotype combinations | Depends on mating behavior | Alters genotype frequencies and can influence selection |
These processes frequently operate simultaneously. A mutation may introduce a new allele. Natural selection may favor it under one environmental condition, while genetic drift may alter its frequency simply through chance. Individuals may move between populations and bring the allele elsewhere through gene flow, and mate choice may determine which genetic combinations become common.
Evolution is therefore not the result of a single mechanism. It is the combined outcome of processes that alter genetic variation, its distribution, and its transmission from one generation to the next.
