Evolution is the change in the heritable characteristics of biological populations over successive generations. Historically, evolutionary theory focused primarily on how organisms adapt to their environments, driven by forces that favor fitter individuals. This perspective suggested that most genetic differences directly impacted survival and reproductive success. However, a significant portion of genetic change occurs without relationship to adaptation or environmental pressure. This non-adaptive component is explained by neutral evolution, where molecular changes are governed by pure chance rather than fitness advantages.
Defining Neutral Evolution
The Neutral Theory of Molecular Evolution proposes that most genetic variation within a species’ DNA, and the differences that accumulate between species, are due to mutations that have no effect on an organism’s ability to survive or reproduce. These genetic alterations are termed “neutral mutations” because they are functionally silent, neither benefiting nor harming the individual’s fitness. The theory holds that natural selection’s influence is negligible on these specific molecular changes.
Neutrality often arises due to the degenerate nature of the genetic code. Multiple three-letter DNA sequences (codons) can code for the same amino acid. A DNA change resulting in the same amino acid being incorporated into a protein is a synonymous substitution, which typically does not alter the protein’s function. Changes in non-coding regions of the genome are also frequently neutral.
Japanese geneticist Motoo Kimura formally introduced this concept in the late 1960s, radically reshaping the understanding of molecular evolution. Kimura postulated that while deleterious mutations are quickly removed by selection and beneficial mutations are rare, the vast majority of new mutations are neutral. The fate of these neutral changes is determined by statistical accident, not the environment.
The Mechanism: Genetic Drift
Genetic drift is the purely random mechanism responsible for the spread and fixation of neutral mutations. It involves the fluctuation of allele frequencies—the relative proportion of a specific gene variant—from one generation to the next due to chance events in survival and reproduction. Because populations are finite, only a random sample of parental genes passes to the next generation. This sampling error allows an allele’s frequency to randomly increase or decrease, regardless of its effect on fitness.
The strength of genetic drift is inversely related to population size. In large populations, random sampling effects are minimal because a greater number of individuals buffers against chance fluctuations. Conversely, in small populations, a random event affecting a few individuals can drastically alter the group’s genetic makeup. Over time, drift will cause a neutral allele to either disappear completely or become “fixed,” reaching 100% frequency.
Demographic events can dramatically increase the power of genetic drift.
Bottleneck Effect
The bottleneck effect occurs when a large population is suddenly reduced to a small size by a severe, non-selective event, such as a natural disaster. The small group of survivors carries only a fraction of the original genetic diversity. Consequently, the resulting population’s gene frequencies may differ significantly from the pre-disaster population.
Founder Effect
The founder effect happens when a small subset of a larger population separates and establishes a new colony in a different location. The genetic makeup of the new population is determined solely by the few founding individuals, often not representing the original population’s diversity. For example, the high prevalence of rare genetic disorders, like Ellis-van Creveld syndrome among the Old Order Amish, illustrates how the founder effect concentrates a rare allele by chance in an isolated population.
Comparing Neutral Evolution and Natural Selection
Neutral evolution and natural selection are the two primary forces that drive evolutionary change, but they operate on fundamentally different principles. Natural selection is a deterministic force, meaning its action is directed by the environment; it consistently favors traits that increase fitness and leads to adaptation. This process acts on the phenotype, shaping traits like camouflage or resistance to disease.
In contrast, neutral evolution, driven by genetic drift, is a stochastic or random process. It lacks direction and does not lead to adaptation because it acts primarily on genotypes—the differences in DNA sequence—where there are no functional differences that affect survival or reproduction. Neutral evolution can just as easily eliminate a slightly beneficial allele as it can fix a slightly harmful one, especially in small populations where chance effects are strong.
A key distinction is that selection is most effective in large populations where even small differences in fitness can be consistently amplified across generations. Genetic drift, however, is the dominant force in small populations and for mutations with negligible effects on fitness, regardless of population size. Evolutionary biologists now recognize that while natural selection is responsible for the adaptive changes observed in morphology and behavior, neutral evolution accounts for the majority of genetic changes observed at the molecular level.
Observable Consequences: The Molecular Clock
A major prediction and practical consequence of the Neutral Theory is the concept of the molecular clock. The theory posits that the rate at which neutral mutations become fixed in a population is constant and equal to the rate at which those mutations arise in the first place, independent of the population size. This constant rate exists because the chances of a neutral mutation arising and the chances of it being fixed by drift cancel each other out.
This predictable, clock-like accumulation of neutral substitutions allows scientists to use genetic differences to estimate the time of divergence between species. By comparing the number of sequence differences in a given gene between two species and calibrating that rate using fossil evidence or known separation times, researchers can establish a molecular timeline. The molecular clock has become a powerful tool in phylogenetics, providing a method to construct evolutionary trees and estimate when different lineages split, based on the steady, random ticking of neutral genetic change.