Balancing Selection

Definition

Balancing selection is an evolutionary mechanism that maintains genetic variation within a population by favoring the persistence of multiple alleles at a given locus. It occurs when heterozygous individuals have a higher fitness than either homozygous type, leading to the maintenance of polymorphism in the population.

5 Must Know Facts For Your Next Test

1. Balancing selection maintains genetic variation by favoring the persistence of multiple alleles at a given locus in a population.
2. Heterozygote advantage, where heterozygous individuals have higher fitness than either homozygous type, is a common mechanism of balancing selection.
3. Frequency-dependent selection, where the fitness of a genotype depends on its frequency relative to other genotypes, can also lead to balancing selection.
4. Overdominance, where the heterozygous genotype has a higher fitness than either homozygous genotype, is a specific form of balancing selection.
5. Balancing selection can be important in maintaining diversity in genes related to immune function, such as the major histocompatibility complex (MHC) in humans.

Review Questions

• Explain how balancing selection maintains genetic variation within a population.
  • Balancing selection maintains genetic variation by favoring the persistence of multiple alleles at a given locus. This can occur through mechanisms like heterozygote advantage, where individuals with a heterozygous genotype have higher fitness than those with either homozygous genotype. Balancing selection prevents the loss of genetic diversity and ensures that a variety of alleles are maintained in the population over time.
• Describe the role of frequency-dependent selection in balancing selection.
  • Frequency-dependent selection is a type of balancing selection where the fitness of a genotype depends on its frequency relative to other genotypes in the population. When a rare genotype has a higher fitness than more common genotypes, this can lead to the maintenance of genetic variation. As the rare genotype becomes more common, its fitness advantage may decrease, allowing other genotypes to persist in the population. This dynamic contributes to the long-term maintenance of multiple alleles at a given locus.
• Analyze how balancing selection, such as through overdominance, can be important in maintaining diversity in genes related to immune function.
  • Balancing selection, particularly through the mechanism of overdominance, where the heterozygous genotype has a higher fitness than either homozygous genotype, can be crucial in maintaining diversity in genes related to immune function. For example, in the case of the major histocompatibility complex (MHC) in humans, a diverse array of MHC alleles allows the immune system to recognize and respond to a wider range of pathogens. Individuals with a heterozygous MHC genotype may have a stronger immune response and higher fitness compared to those with homozygous MHC genotypes. This balancing selection preserves MHC diversity within the population, enhancing the overall immune capabilities of the species.

Related terms

Heterozygote Advantage: A situation where individuals with a heterozygous genotype have a higher fitness than those with either homozygous genotype, leading to the maintenance of genetic variation.

Frequency-Dependent Selection: A type of balancing selection where the fitness of a genotype depends on its frequency relative to other genotypes in the population.

Overdominance: A form of balancing selection where the heterozygous genotype has a higher fitness than either homozygous genotype.