Why Do Lethal Mutations Persist in Populations? The Mutation-Selection Balance Explained

Genetic mutations are a natural part of evolution, but not all mutations are beneficial. Some are harmful—or even lethal—yet small amounts of these lethal mutations are consistently found in populations. Why do these detrimental alleles persist, instead of being entirely eliminated by natural selection? The answer lies in a fundamental concept of evolutionary genetics: mutation-selection balance.

What Is Mutation-Selection Balance?

Mutation-selection balance is the equilibrium that arises when the rate at which new deleterious (including lethal) mutations are introduced into a population by mutation is balanced by the rate at which they are removed by natural selection. In other words, while selection acts to eliminate harmful alleles, new copies of these alleles are continuously introduced by mutation.

How Does It Work?

• Mutation introduces new lethal alleles into the gene pool each generation.

• Natural selection removes these alleles by reducing the survival or reproductive success of individuals carrying them.

• At equilibrium, the frequency of the lethal allele in the population remains low but nonzero, because the influx of new mutations matches the rate at which selection removes them.

This balance explains why, even in large populations, completely eliminating all lethal mutations is virtually impossible. As soon as one copy is removed, another may arise through mutation.

Real-World Examples

• Many recessive genetic disorders in humans, such as Tay-Sachs disease or cystic fibrosis, persist at low frequencies in populations due to this balance.

• The equilibrium frequency depends on the mutation rate and the strength of selection against the allele.

Scientific Support

Research and theoretical models consistently show that the mutation-selection balance is a key mechanism maintaining small amounts of lethal mutations in populations. While other forces like balancing selection or genetic drift can play roles in specific cases, the baseline persistence of lethal mutations is best explained by this equilibrium.

Correct Answer

Small amounts of lethal mutation always tend to remain in a population due to:

(1) Mutation-Selection balance

This principle is fundamental to understanding genetic variation and the persistence of deleterious alleles in natural populations.