Microevolution is the study of evolutionary changes occurring on the smallest possible scale: within a single population or species over a short duration. It is defined by a change in the frequency of alleles—the different versions of a gene—from one generation to the next. These measurable shifts in a population’s gene pool represent evolution happening in real-time, often occurring over just a few successive generations. Researchers track these genetic fluctuations to understand how organisms adapt and change in response to their environment.
Microevolution Versus Macroevolution
Microevolution and macroevolution describe the same fundamental process—genetic change over time—but differ significantly in scope, scale, and timeframe. Microevolution focuses strictly on genetic shifts within a species or population, such as a change in the prevalence of a specific coat color allele in a mouse colony, observable over brief periods. Macroevolution, by contrast, refers to large-scale evolutionary patterns that occur above the species level, involving the emergence of entirely new species or higher taxonomic groups.
Macroevolutionary change is the result of microevolutionary changes accumulating over vast expanses of geological time. A series of small, measurable changes in allele frequencies eventually lead to reproductive isolation. This results in a population genetically distinct enough to be classified as a new species.
Forces That Drive Allele Change
The gene pool of a population can be altered by four distinct mechanisms that drive microevolutionary change.
Natural Selection
Natural selection is the process where individuals with certain inherited traits tend to survive and reproduce more successfully than others in a specific environment. This differential reproductive success increases the frequency of beneficial alleles in the next generation. For instance, if a brown rabbit allele provides better camouflage against predators than a white rabbit allele, the brown allele will become more common over successive generations.
Genetic Drift
Genetic drift involves random fluctuations in allele frequencies that occur purely by chance, irrespective of the trait’s benefit to fitness. This effect is pronounced in small populations where random events, like the failure of a few individuals to reproduce, disproportionately impact the gene pool. The bottleneck effect is one form of drift where a catastrophic event drastically reduces population size, leaving a small, random assortment of survivors whose allele frequencies are often unrepresentative of the original population.
Gene Flow
Gene flow describes the movement of alleles between populations through the migration and subsequent interbreeding of individuals. When individuals from one population successfully mate with individuals in another, they introduce new alleles into that population’s gene pool. This genetic transfer increases the diversity of the recipient population and makes separate populations more genetically similar.
Mutation
Mutation is the ultimate source of all new genetic variation upon which the other mechanisms can act. Mutations are random, spontaneous errors in the DNA replication process that result in new alleles. While most mutations are harmful or neutral, a rare few can confer a survival or reproductive advantage. Mutation adds variation to the gene pool, providing the raw material for future evolutionary change.
Observing Microevolution in Action
Microevolution is constantly occurring and can be observed in controlled studies and real-world scenarios.
Antibiotic Resistance
A clear example of natural selection is the rapid development of antibiotic resistance in bacteria. When an antibiotic is introduced, it acts as an intense selective pressure, quickly eliminating susceptible bacterial cells. Any bacterium that possesses a chance mutation—such as one that modifies the drug’s target—survives and reproduces. This results in a new population dominated by resistant strains.
The Peppered Moth
Another illustration of directional selection is the peppered moth (Biston betularia) in Britain during the Industrial Revolution. Before industrialization, the light-colored form was prevalent because it was camouflaged against lichen-covered trees. As soot darkened the trunks, the rare, dark-colored (melanic) moths became better camouflaged. The resulting survival advantage meant that the allele for dark coloration rapidly increased in frequency across polluted areas.
Artificial Selection
Microevolution is also evident in artificial selection, where humans intentionally manipulate the genetic makeup of species for desired traits. For thousands of years, humans have selectively bred wolves to create the numerous dog breeds seen today, a process that rapidly alters allele frequencies. Similarly, agriculture relies on selecting plants with traits like higher yield or pest resistance, dramatically changing the genetic composition of crop species.
Why Understanding Microevolution Matters
Understanding the mechanisms of microevolution is relevant to solving practical challenges in public health, conservation, and food security.
Public Health
The struggle against infectious diseases is an evolutionary race, where pathogens constantly evolve resistance to medications. Tracking the genetic mechanisms of resistance in organisms like Mycobacterium tuberculosis is essential for developing effective, long-term treatment strategies.
Conservation Biology
In conservation biology, understanding genetic drift is important for managing threatened or endangered species. Small, isolated populations are susceptible to genetic bottlenecks, which reduce diversity and leave the species vulnerable to disease or environmental change. Conservation efforts often focus on restoring gene flow between fragmented populations to replenish genetic variation and mitigate the negative effects of drift.
Agriculture
The principles of microevolution are applied to agriculture to protect global food sources. Pests, weeds, and pathogens evolve resistance to pesticides, herbicides, and fungicides through natural selection. By understanding the genetic basis and rate of this evolution, scientists can rotate chemical treatments and implement integrated pest management strategies to slow the development of resistance.