Evolution refers to the change in the heritable characteristics of biological populations across successive generations. This fundamental process shapes the diversity of life on Earth. Two primary mechanisms driving these changes are natural selection and genetic drift. While they operate through distinct forces—one adaptive and the other random—these two evolutionary processes share important similarities often overlooked.
Shared Evolutionary Outcome: Altering Allele Frequencies
A core outcome shared by both natural selection and genetic drift is their ability to alter allele frequencies within a population. Alleles are different versions of a gene, and their frequencies represent the proportion of a specific gene variant present in a population’s gene pool. Evolution, at its most basic level, is defined as any shift in these allele frequencies over time.
Natural selection influences allele frequencies based on the reproductive success of individuals. Organisms possessing traits that provide an advantage in a given environment are more likely to survive, reproduce, and pass on the alleles associated with those beneficial traits to their offspring. This differential survival and reproduction leads to an increase in the frequency of advantageous alleles and a decrease in less favorable ones across generations.
In contrast, genetic drift causes changes in allele frequencies due to random chance events. This can be thought of as a “sampling error” in which alleles are passed from one generation to the next. For example, random events like some individuals having more offspring or dying for non-genetic reasons can shift allele proportions in the next generation.
Impact on Genetic Diversity Within Populations
Both natural selection and genetic drift can lead to a reduction in the overall genetic variation or diversity within a population. Genetic diversity represents the range of different alleles present for various genes. A reduction in this diversity can limit a population’s capacity to adapt to changing environmental conditions.
Natural selection reduces genetic diversity by favoring advantageous alleles, causing them to become more common, while less beneficial or disadvantageous alleles become rarer or are eliminated from the population. This process actively removes genetic variations that are not well-suited for survival and reproduction in a given environment.
Genetic drift, on the other hand, reduces genetic diversity through random sampling. In each generation, only a subset of the alleles from the parental generation is passed on, and by chance, some alleles may not be transmitted at all. This random loss can lead to the disappearance of certain alleles, regardless of whether they are beneficial, neutral, or harmful. Both processes can result in “fixation,” where an allele reaches 100% prevalence in the population, or “loss,” where it becomes completely absent.
Influence of Population Size
The impact of both natural selection and genetic drift is significantly influenced by population size. Genetic drift, in particular, exerts a much stronger and more rapid effect in smaller populations. In a small population, random chance events have a proportionally larger impact on allele frequencies because each individual’s genetic contribution to the next generation represents a greater fraction of the total gene pool.
For example, if a population has only a few individuals, the random failure of one individual to reproduce can lead to the complete loss of their unique alleles, significantly altering the population’s genetic composition. This increased susceptibility to random fluctuations means that alleles can quickly become fixed or lost, regardless of their adaptive value.
While natural selection operates in populations of all sizes, its effects can be more easily masked by the stronger influence of genetic drift in small populations. Beneficial mutations might be lost by chance before selection can act upon them effectively. Conversely, in very large populations, the effects of genetic drift are considerably less significant due to the “law of large numbers,” and natural selection can operate more consistently and efficiently to sort beneficial from detrimental traits.