Which Type Of Selection Tends To Increase Genetic Variation

Genetic variation, the cornerstone of evolutionary adaptation, fuels the ability of populations to respond to changing environmental pressures. While selection often drives the reduction of genetic diversity by favoring specific traits, certain types of selection actively promote and maintain it. Understanding these selective forces is crucial for comprehending the dynamics of evolution and the resilience of populations. This article delves into the specific modes of selection that tend to increase genetic variation, providing a comprehensive overview of their mechanisms and ecological significance.

Balancing Selection: Maintaining Diversity

Balancing selection stands out as a primary force driving the increase and maintenance of genetic variation within populations. Unlike directional selection, which favors one extreme phenotype, or stabilizing selection, which favors the intermediate phenotype, balancing selection actively maintains multiple alleles at a locus, preventing the fixation of any single allele. Several mechanisms contribute to balancing selection:

1. Heterozygote Advantage

Heterozygote advantage, also known as overdominance, occurs when individuals with heterozygous genotypes exhibit higher fitness than either homozygous genotype. This phenomenon directly promotes genetic diversity by favoring the maintenance of both alleles within the population.

Mechanism:

• Increased Functional Capacity: In some cases, the combination of two different alleles in a heterozygote results in a protein or functional pathway with enhanced activity or broader substrate specificity. This can lead to improved performance under a wider range of conditions.
• Resistance to Multiple Pathogens: Heterozygotes may possess resistance to multiple pathogens, whereas homozygotes are susceptible to at least one. This is particularly relevant in environments with high pathogen diversity.
• Avoidance of Deleterious Recessive Alleles: Heterozygote advantage can mask the effects of deleterious recessive alleles. While homozygotes for the recessive allele suffer reduced fitness, heterozygotes carrying one copy of the recessive allele and one copy of the dominant allele do not express the deleterious phenotype and may even experience increased fitness.

Examples:

• Sickle Cell Anemia: Perhaps the most well-known example of heterozygote advantage is the case of sickle cell anemia. The sickle cell allele (HbS) causes red blood cells to deform into a sickle shape, leading to various health problems in homozygous individuals (HbS/HbS). However, heterozygotes (HbA/HbS) carrying one normal allele (HbA) and one sickle cell allele exhibit resistance to malaria. In regions where malaria is prevalent, the heterozygote advantage maintains both the HbA and HbS alleles in the population.
• Cystic Fibrosis: While cystic fibrosis (CF) is a severe genetic disorder caused by mutations in the CFTR gene, studies suggest that heterozygotes carrying one copy of the mutated gene may have increased resistance to certain infectious diseases, such as typhoid fever. This could potentially explain the relatively high frequency of CFTR mutations in some populations.
• MHC Genes: Major Histocompatibility Complex (MHC) genes play a crucial role in the immune system by encoding proteins that present antigens to T cells. Heterozygosity at MHC loci is often associated with enhanced immune responses and broader resistance to pathogens. Individuals with diverse MHC alleles can recognize a wider range of antigens, providing better protection against a variety of infections.

2. Frequency-Dependent Selection

Frequency-dependent selection occurs when the fitness of a genotype depends on its frequency in the population. This type of selection can maintain genetic diversity by favoring rare genotypes.

Mechanism:

• Negative Frequency-Dependent Selection: In negative frequency-dependent selection, the fitness of a genotype decreases as it becomes more common. This prevents any single genotype from becoming fixed and promotes the maintenance of multiple alleles.
• Predator-Prey Interactions: Predators may develop a search image for the most common prey type, leading to increased predation pressure on that type. Rare prey types, on the other hand, may experience lower predation rates, giving them a fitness advantage.
• Parasite-Host Interactions: Parasites often evolve to exploit the most common host genotype. Rare host genotypes may be resistant to the parasite, giving them a fitness advantage.
• Competition: In competitive interactions, rare genotypes may have access to resources that are not readily available to the more common genotypes. This can give them a competitive advantage and promote their persistence in the population.

Examples:

• Scale-Eating Fish: Scale-eating fish in Lake Tanganyika exhibit frequency-dependent selection on their mouth morphology. These fish have either left-sided or right-sided mouths, which they use to scrape scales off their prey. When one mouth morph becomes more common, their prey become more vigilant on that side of their body, giving the rarer mouth morph an advantage.
• Self-Incompatibility in Plants: Many plant species have self-incompatibility systems that prevent self-fertilization. This promotes outcrossing and maintains genetic diversity by favoring rare self-incompatibility alleles. When a pollen grain lands on a stigma with the same self-incompatibility allele, fertilization is blocked. Rare alleles have a higher chance of successful fertilization because they are less likely to encounter a matching allele.

3. Spatially Varying Selection

Spatially varying selection occurs when different genotypes are favored in different environments or habitats. This can maintain genetic diversity across the species range by preventing the fixation of any single genotype.

Mechanism:

• Environmental Heterogeneity: Different environments may favor different traits. For example, plants growing in arid environments may be selected for drought tolerance, while plants growing in wet environments may be selected for flood tolerance.
• Gene Flow: Gene flow between populations in different environments can introduce new alleles and maintain genetic diversity across the species range. However, gene flow can also counteract the effects of local selection by introducing maladaptive alleles.
• Local Adaptation: Local adaptation occurs when populations evolve to be better suited to their local environment. This can lead to the evolution of distinct ecotypes or subspecies.

Examples:

• Heavy Metal Tolerance in Plants: Plants growing in soils contaminated with heavy metals, such as copper or zinc, may evolve tolerance to these metals. This can lead to the formation of distinct ecotypes that are adapted to either contaminated or uncontaminated soils.
• Shell Color in Snails: Shell color in snails can vary depending on the background color of their habitat. Snails with shell colors that match their background are better camouflaged from predators and have a higher survival rate.
• Industrial Melanism in Moths: The peppered moth is a classic example of spatially varying selection. During the Industrial Revolution, pollution caused the bark of trees to darken, giving dark-colored moths a camouflage advantage over light-colored moths. In unpolluted areas, light-colored moths remained more common.

Disruptive Selection: Favoring Extremes

Disruptive selection, also known as diversifying selection, favors individuals with extreme phenotypes over individuals with intermediate phenotypes. This can lead to the evolution of distinct subpopulations or even new species.

Mechanism:

• Bimodal Fitness Landscape: Disruptive selection creates a bimodal fitness landscape, where the highest fitness values are associated with the extreme phenotypes and the lowest fitness values are associated with the intermediate phenotypes.
• Resource Competition: Disruptive selection can occur when resources are limited and individuals with different resource utilization strategies have a competitive advantage.
• Habitat Heterogeneity: Disruptive selection can also occur when the environment is heterogeneous and different phenotypes are favored in different habitats.

Examples:

• Black-Bellied Seedcrackers: Black-bellied seedcrackers in Cameroon exhibit disruptive selection on beak size. These birds feed on seeds of different sizes, and birds with either large or small beaks are more efficient at feeding on their respective seed sizes than birds with intermediate beak sizes.
• Three-Spined Sticklebacks: Three-spined sticklebacks in some lakes exhibit disruptive selection on body size. These fish occupy two different niches: benthic (bottom-dwelling) and limnetic (open-water). Benthic sticklebacks are typically larger and have deeper bodies, while limnetic sticklebacks are typically smaller and have more streamlined bodies.
• Darwin's Finches: Darwin's finches on the Galapagos Islands are a classic example of adaptive radiation. These finches have evolved a variety of beak shapes and sizes to exploit different food sources. Disruptive selection has played a role in the diversification of beak morphology in these finches.

Mutation-Selection Balance: A Constant Source of Variation

While mutation is the ultimate source of all genetic variation, its interaction with selection plays a critical role in maintaining diversity, especially for deleterious alleles.

Mechanism:

• Introduction of New Alleles: Mutation constantly introduces new alleles into the population, including deleterious alleles.
• Selection Against Deleterious Alleles: Selection acts to remove deleterious alleles from the population.
• Equilibrium: The balance between the introduction of new deleterious alleles by mutation and the removal of deleterious alleles by selection results in an equilibrium frequency of deleterious alleles in the population.

Examples:

• Genetic Disorders: Many genetic disorders are caused by deleterious recessive alleles. While selection acts to reduce the frequency of these alleles, mutation constantly introduces new copies, resulting in a low but persistent frequency of these alleles in the population.
• Drug Resistance: The evolution of drug resistance in pathogens is often driven by mutation. While drug-resistant strains may have a fitness cost in the absence of the drug, the mutation rate and selection pressure in the presence of the drug can lead to the rapid spread of resistance alleles.

The Interplay of Selection and Other Evolutionary Forces

It's crucial to recognize that selection doesn't operate in isolation. Other evolutionary forces, such as genetic drift, gene flow, and non-random mating, also influence genetic variation. The interplay between these forces can lead to complex evolutionary outcomes.

• Genetic Drift: Genetic drift, the random fluctuation of allele frequencies, can lead to the loss of genetic variation, especially in small populations. However, in some cases, genetic drift can also maintain genetic variation by preventing the fixation of beneficial alleles.
• Gene Flow: Gene flow, the movement of genes between populations, can introduce new alleles and increase genetic variation within a population. However, gene flow can also counteract the effects of local selection by introducing maladaptive alleles.
• Non-Random Mating: Non-random mating, such as assortative mating (mating with individuals with similar phenotypes), can alter genotype frequencies and influence genetic variation.

Conclusion

While selection is often perceived as a force that reduces genetic variation, certain types of selection, particularly balancing selection and disruptive selection, actively promote and maintain it. Heterozygote advantage, frequency-dependent selection, and spatially varying selection are key mechanisms driving balancing selection. Disruptive selection favors extreme phenotypes and can lead to the evolution of distinct subpopulations. Furthermore, the interplay between mutation and selection ensures a constant influx of new genetic variants, while the balance between these forces maintains a certain level of genetic diversity. Understanding the various modes of selection and their interaction with other evolutionary forces is crucial for comprehending the complex dynamics of evolution and the resilience of populations in the face of environmental change. By maintaining genetic variation, these selective forces ensure that populations have the raw material necessary to adapt to future challenges.