The key difference between stabilizing and balancing selection is that stabilizing selection is a type of natural selection which favours average phenotypes in a population while balancing selection is the maintenance of multiple alleles of a gene within a population in order to enhance genetic diversity.
Stabilizing selection is a type of natural selection that applies to a phenotypic trait. It favours average phenotypes in a population. Therefore, it eliminates both types of extreme phenotypes. Ultimately it makes a uniform population. Balancing selection is several selective processes that maintain advantageous genetic diversity within a population by maintaining multiple alleles of a gene. Therefore, it applies to a gene locus.
CONTENTS
1. Overview and Key Difference
2. What is Stabilizing Selection
3. What is Balancing Selection
4. Similarities Between Stabilizing and Balancing Selection
5. Side by Side Comparison – Stabilizing vs Balancing Selection in Tabular Form
6. Summary
What is Stabilizing Selection?
Stabilizing selection is a type of natural selection that favours the average individuals in a population. In other words, stabilizing selection pushes a population towards the average or median while eliminating the two extreme phenotypes. The environment normally favours the average phenotype within the population. Stabilizing selection is the quantitative equivalent of balancing selection for a single gene trait.
Figure 01: Stabilizing Selection
For example, birth weight in human shows stabilizing selection against very small and very large birth weights. Another example is the body size of a species of lizard belonging to the genus Aristelliger. Small lizards and large lizards are eliminated, and the average size lizards are favoured by the natural selection. Stabilizing selection makes the population more uniform since natural selection works against the two extremes.
What is Balancing Selection?
Balancing selection is the maintenance of two or more alleles in a population. It maintains advantageous genetic diversity within populations. There are two key mechanisms for balancing selection. They are heterozygote advantage and frequency-dependent selection. Both of them lead to a stable polymorphic equilibrium state. Heterozygotes show higher relative fitness than both homozygotes, leading to balanced polymorphism. Therefore, the organism will have both alleles of the gene rather than having two copies of either version alone. This brings about a heterozygote advantage.
In frequency-dependent selection, the reproductive success of a phenotype depends on the frequency, especially when it has a lower frequency. Lower the frequency, higher the fitness, leading to a balanced polymorphism. Fitness of a phenotype decreases as it becomes more common. Therefore rare phenotypes show higher fitness and are favoured by the selection. This negative frequency-dependent selection leads to a balanced polymorphism.
What are the Similarities Between Stabilizing and Balancing Selection?
- Stabilizing selection is the quantitative equivalent of balancing selection.
- Both are advantageous for the balance of populations.
What is the Difference Between Stabilizing and Balancing Selection?
Stabilizing selection is a type of selection which eliminates both extremes from an array of phenotypes while balancing selection is a number of selective processes that actively maintain multiple alleles of a gene in the gene pool of a population. So, this is the key difference between stabilizing and balancing selection. In stabilizing selection, the mean value of a phenotype is selected on a phenotypic trait while in balancing selection, multiple alleles of a gene are selected.
Moreover, stabilizing selection makes a uniform population while balancing selection concerns the genetic polymorphism.
Below infographic summarizes the difference between stabilizing and balancing selection in tabular form.
Summary – Stabilizing vs Balancing Selection
Stabilizing and balancing selection are two types of selection mechanisms occurring in populations. Stabilizing selection is applied to a phenotypic trait while balancing selection is applied to a given locus. Stabilizing selection is a type of natural selection which favours average phenotypes within a population while eliminating both extreme phenotypes. Balancing selection refers to several mechanisms that maintain multiple alleles of a gene within a population. Balancing selection contributes to the increased genetic diversity via two key mechanisms: heterozygote advantage and frequency-dependent selection. Thus, this is the summary of the difference between stabilizing and balancing selection.
Reference:
1. “Balancing Selection”. En.Wikipedia.Org, 2020, Available here.
Image Courtesy:
1. “Directional, Disruptive and Stabilizing Selections” By Ibbiologystudentone – Own work (CC BY-SA 4.0) via Commons Wikimedia