Factors Driving Microevolution

The biologist Richard Goldschmidt coined the term microevolution in 1940 to capture a specific idea: small mutations accumulating within a population over time, enough to give rise to new sub-species and races, but not enough to produce an entirely new species. If that is what microevolution describes, the natural follow-up question is: what actually pushes those changes along? Five distinct forces work behind the scenes to shift gene frequencies in a population. Some introduce new variation, others filter it, and still others shuffle it around by chance. Together, they explain how populations gradually diverge without ever crossing the line into a completely different species.

Mutation: The Raw Material of All Change

Every other evolutionary force depends on genetic variation already being present. Mutation (a random change in the DNA sequence) is the only process that creates that variation in the first place. Mutations produce small, subtle changes in an organism’s observable traits. These changes are typically modest enough that they do not wipe out the survival advantages a population has built up over thousands of years.

Consider the adaptations that different human populations have developed for dealing with temperature extremes. The Eskimos of the Arctic, the Peruvians of the high Andes, the Nepalese of the Himalayan foothills, and the peoples of tropical Africa each carry distinct sets of heat and cold adaptations. These differences trace back to small mutations that accumulated independently in each population over many generations, gradually fine-tuning their physiology for local conditions.

Not all mutations are equal, though. Some regions of the genetic code are far more sensitive to change than others. Molecular genetics research has revealed that mutations in critical areas of the genome are almost never tolerated. A clear example involves histone proteins (the structural proteins around which DNA coils to form chromosomes). Because histones perform such a vital function in packaging genetic material, even tiny alterations to them can be catastrophic. Natural selection weeds out histone mutations almost immediately. By contrast, small mutations in less critical regions of DNA, those outside the core functional domains, are more likely to survive and spread through a population.

Natural Selection: The Filter That Shapes Variation

Once mutations introduce new variants into a population, natural selection determines which variants persist and which disappear. Researchers study natural selection through three main approaches:

• Molecular genetics — By comparing DNA sequences across species, scientists can identify which parts of the genome resist change (because mutations there are lethal) and which parts tolerate variation. The histone protein example above falls into this category.
• Trait distribution — Some traits show striking geographic patterns that line up with environmental pressures. The sickle cell trait is a well-known case: it persists at high frequencies across the malaria belt of Africa because carriers gain protection against the disease. The geographic overlap between the trait and the disease zone is strong evidence that natural selection is maintaining it.
• Single population studies — Researchers sometimes focus on one specific group to trace how selection has shaped its gene frequencies. The Gorkhas of Nepal, for instance, have been studied for genes that provide adaptation to the cold, high-altitude climate of the Himalayan region.

Three Modes of Natural Selection

Natural selection does not always work in the same way. Depending on the environmental pressures at play, it can push a population’s traits in three very different directions:

Stabilizing selection favours the average. Individuals at both extremes of a trait are selected against, while those near the middle survive and reproduce most successfully. Picture a bell curve: stabilizing selection narrows the curve by trimming both tails. The population becomes more uniform, clustering tightly around the mean. This is the most common mode and is typical of stable environments where the existing average is already well adapted.

Directional selection pushes the entire population toward one extreme. When environmental conditions change and favour a trait at one end of the spectrum, individuals with that trait survive and reproduce more successfully. Over time, the population mean shifts in that direction. Visually, the bell curve slides to one side.

Disruptive selection is the opposite of stabilizing. It favours both extremes simultaneously while selecting against the average. This creates a split in the population, producing a two-peaked (bimodal) distribution. If this continues long enough, the population may eventually divide into two distinct groups, potentially setting the stage for the formation of new species.

Gene Flow: Genes on the Move

Gene flow is the movement of genetic material from one population to another. Whenever individuals migrate from one group and reproduce in a new one, they carry their alleles (gene variants) with them. This transfer introduces new genetic variation into the receiving population and can change its gene frequencies.

The strength of gene flow depends on the scale of migration. When large numbers of individuals move between populations, gene flow is strong and the two populations tend to become more genetically similar over time. When migration is minimal, populations are free to diverge.

Genetic Drift: When Chance Takes the Wheel

Genetic drift is random change in gene frequency that has nothing to do with whether a trait is helpful, harmful, or neutral. It happens purely by chance, and its effects are most powerful in small populations.

In a large population, random fluctuations tend to cancel each other out across thousands of individuals. In a small population, however, a single chance event, such as a natural disaster killing off a random subset of individuals, or the founding of a new colony by just a handful of people, can dramatically alter allele frequencies.

Two well-documented examples illustrate this:

• Six-fingered dwarfism in the Old Order Amish — The Amish communities of Pennsylvania descend from a small group of European founders. By chance, one or more of those founders carried the allele for Ellis-van Creveld syndrome (a condition causing extra fingers and short stature). Because the community remained small and intermarried within itself, this rare allele reached an unusually high frequency, far higher than in the general population.
• Retinitis pigmentosa in Tristan da Cunha — Tristan da Cunha is one of the most remote inhabited islands on Earth, located in the South Atlantic. Its tiny, isolated population has an abnormally high rate of retinitis pigmentosa (a degenerative eye condition). The allele responsible offers no survival benefit; it simply drifted to a high frequency because the population was so small that random chance, rather than natural selection, dominated the outcome.

Isolation and Hybridization: Pulling Apart and Bringing Together

The final pair of forces works in opposite directions.

Isolation occurs when populations become separated, whether by geography, behaviour, or some other barrier that prevents interbreeding. Once isolated, each population accumulates its own unique set of mutations and faces its own distinct selection pressures. Over time, the two groups diverge genetically. If isolation persists long enough, they may become so different that they can no longer interbreed even if brought back together, a milestone known as reproductive isolation (the point at which two populations can no longer produce fertile offspring with each other).

Hybridization works in the opposite direction. When individuals from genetically distinct populations do interbreed, their offspring carry a mix of alleles from both parent groups. This blending of gene pools counteracts divergence and prevents reproductive isolation from taking hold. As long as some interbreeding continues between two populations, they remain part of a shared genetic network.

Isolation pushes populations apart; hybridization pulls them back together. The balance between these two forces at any given time determines whether a population is on a path toward becoming a separate species or staying connected to its neighbours.

Microevolution Has Not Stopped

It would be a mistake to think of microevolution as something that happened only in the distant past. The process is still unfolding in living human populations right now.

A simple and relatable example is the wisdom tooth. In earlier human populations, wisdom teeth typically erupted by the age of 16 to 18. Early humans relied on these extra molars to grind tough seeds, roots, and raw plant material. As human diets shifted toward softer, cooked foods, the functional need for wisdom teeth gradually disappeared. Today, many people find that their wisdom teeth appear only in their twenties, or never emerge at all. This shift reflects a slow, ongoing change in human biology driven by the removal of the selective pressure that once kept wisdom teeth reliably present, a quiet but real instance of microevolution at work.