Which Type Of Selection Tends To Increase Genetic Variation

Genetic variation, the raw material for evolution, fuels adaptation and resilience within populations. Understanding the forces that shape this variation is crucial for comprehending evolutionary processes. While natural selection often reduces genetic variation by favoring specific traits, certain types of selection can paradoxically promote or maintain it. This article delves into the fascinating realm of selection pressures that contribute to the increase of genetic variation, exploring mechanisms like disruptive selection, frequency-dependent selection, and spatially varying selection.

Disruptive Selection: Favoring Extremes

Disruptive selection, also known as diversifying selection, is a potent force in increasing genetic variation. It occurs when the extremes of a phenotypic range are favored over intermediate values. Imagine a population of birds where beak size is under selection. If birds with either very small beaks (suited for picking up tiny seeds) or very large beaks (capable of cracking hard nuts) have higher survival and reproduction rates than those with medium-sized beaks, disruptive selection is at play.

• Mechanism: This selection pressure leads to the divergence of the population into distinct groups, each adapted to a specific extreme environment or resource. The intermediate phenotypes, being less well-suited to either extreme, are selected against, leading to their decline in frequency.

• Examples:

  • African Seedcrackers: These birds exhibit a bimodal distribution of beak sizes, driven by the availability of hard and soft seeds.
  • Rock Pocket Mice: In certain habitats, dark-colored mice on dark lava rock and light-colored mice on light-colored sand are favored, leading to two distinct color morphs.

• Genetic Basis: Disruptive selection can lead to the maintenance of multiple alleles at a locus. In the beak size example, alleles that promote small beaks and alleles that promote large beaks will be maintained in the population, even though the heterozygous individuals with intermediate beak sizes are less fit.

Frequency-Dependent Selection: The Rarity Advantage

Frequency-dependent selection is another mechanism that can maintain or increase genetic variation. In this scenario, the fitness of a phenotype depends on its frequency in the population. Typically, this involves negative frequency-dependent selection, where rare phenotypes have a selective advantage.

• Mechanism: When a phenotype is rare, it experiences less competition and/or predation compared to common phenotypes. As its frequency increases, the advantage diminishes, and the selective pressure may even reverse. This dynamic prevents any single phenotype from becoming dominant and maintains diversity.

• Examples:

  • Scale-Eating Fish: These fish have mouths that are either twisted to the left or to the right, allowing them to attack their prey (other fish) from one side. The rarer mouth morphology has an advantage because the prey are less vigilant on that side. As the frequency of one morph increases, the prey become more aware, giving the other morph an advantage.
  • Self-Incompatibility in Plants: Many plant species have mechanisms that prevent self-fertilization. Often, this involves genes that determine pollen compatibility. Rare pollen types have a higher chance of successfully fertilizing an egg, leading to a selective advantage.
  • Mimicry: In some cases, rare mimics have an advantage because predators are less likely to encounter and learn to avoid them.

• Genetic Basis: Frequency-dependent selection often leads to the maintenance of multiple alleles, each associated with a different phenotype. The cycling of selective pressures ensures that no single allele is fixed in the population.

Spatially Varying Selection: Local Adaptation

Spatially varying selection occurs when different environments favor different traits. This can happen on a large scale, such as across different continents, or on a small scale, such as within a single field.

• Mechanism: When populations experience different selection pressures in different locations, they can evolve distinct adaptations to their local environments. This leads to genetic divergence between populations.

• Examples:

  • Industrial Melanism in Moths: During the Industrial Revolution, dark-colored moths became more common in polluted areas, while light-colored moths remained more common in unpolluted areas. This was because the dark moths were better camouflaged against the soot-covered trees.
  • Heavy Metal Tolerance in Plants: Plants growing near mine tailings often evolve tolerance to heavy metals in the soil. Plants growing in uncontaminated soil do not experience this selection pressure.
  • Clinal Variation: Gradual changes in allele frequencies along an environmental gradient, such as altitude or latitude, are often due to spatially varying selection.

• Genetic Basis: Spatially varying selection can lead to the evolution of locally adapted genotypes. If gene flow between populations is limited, these genetic differences can persist, leading to increased genetic variation at the regional or species level.

Heterozygote Advantage: The Hybrid Vigor

While not strictly increasing genetic variation, heterozygote advantage maintains existing variation by favoring individuals with heterozygous genotypes. This occurs when individuals with two different alleles at a locus have higher fitness than individuals with homozygous genotypes.

• Mechanism: The heterozygous genotype may confer a benefit that neither homozygous genotype can provide. This can be due to a variety of reasons, such as increased resistance to disease, broader environmental tolerance, or improved metabolic efficiency.

• Examples:

  • Sickle Cell Anemia: Individuals heterozygous for the sickle cell allele are resistant to malaria, while individuals homozygous for the normal allele are susceptible to malaria, and individuals homozygous for the sickle cell allele suffer from sickle cell anemia.
  • Major Histocompatibility Complex (MHC) Genes: These genes play a crucial role in the immune system. Heterozygous individuals have a wider range of immune responses and are therefore more resistant to a wider range of pathogens.

• Genetic Basis: Heterozygote advantage directly maintains multiple alleles at a locus, preventing any single allele from becoming fixed in the population.

Mutation-Selection Balance: The Constant Input

While selection often acts to reduce genetic variation, mutation constantly introduces new variation into the population. The balance between mutation and selection can maintain genetic variation at a certain level.

• Mechanism: Deleterious mutations are constantly arising in the population. Selection acts to remove these mutations, but new mutations continue to arise at a certain rate. The equilibrium frequency of a deleterious allele is determined by the balance between the rate of mutation and the strength of selection.

• Genetic Basis: Mutation introduces new alleles into the population, while selection removes deleterious alleles. The balance between these two forces maintains a certain level of genetic variation.

Gene Flow: Introducing Novel Alleles

Gene flow, the movement of genes between populations, can introduce new alleles into a population, thereby increasing genetic variation.

• Mechanism: When individuals migrate from one population to another and interbreed, they introduce their genes into the recipient population. If the two populations have different allele frequencies, gene flow will increase the genetic variation in the recipient population.

• Examples:

  • Migration of humans: Human populations have migrated extensively throughout history, leading to the mixing of genes between different populations.
  • Pollen dispersal in plants: Pollen can be carried long distances by wind or insects, leading to gene flow between plant populations.

• Genetic Basis: Gene flow introduces new alleles into a population, increasing the genetic variation.

Fluctuating Selection: The Temporal Dance

Fluctuating selection refers to changes in the direction or intensity of selection pressures over time. This can be due to changes in environmental conditions, such as temperature, rainfall, or resource availability.

• Mechanism: When selection pressures fluctuate, different traits may be favored at different times. This can prevent any single trait from becoming fixed in the population and maintain genetic variation.

• Examples:

  • Darwin's Finches: During droughts, finches with larger beaks are favored because they can crack open hard seeds. During wet periods, finches with smaller beaks are favored because they can more easily consume small seeds.
  • Seasonal Variation in Insecticide Resistance: Insecticide resistance may be favored during periods of heavy insecticide use, but disfavored during periods of low insecticide use due to the metabolic costs of resistance.

• Genetic Basis: Fluctuating selection can maintain multiple alleles at a locus if different alleles are favored at different times.

Sexual Selection: The Power of Mate Choice

Sexual selection, a form of natural selection driven by competition for mates, can also contribute to increased genetic variation. This occurs through mechanisms like mate choice, where individuals select partners based on certain traits, and intrasexual competition, where individuals compete with members of their own sex for access to mates.

• Mechanism: Mate choice can lead to the maintenance of genetic variation if females (typically) prefer to mate with males displaying rare or novel traits. Intrasexual competition can favor males with diverse strategies or physical attributes, preventing any single type from dominating.

• Examples:

  • Peacocks' Tails: The elaborate tails of peacocks are thought to be a result of female choice. Females prefer to mate with males with more colorful and symmetrical tails, leading to the evolution of these extravagant displays.
  • Antler Size in Deer: Male deer with larger antlers are more likely to win fights and gain access to mates. This has led to the evolution of large antlers in deer populations.

• Genetic Basis: Sexual selection can maintain genetic variation by favoring rare traits or by selecting for a diversity of traits related to mate acquisition or competition.

Overdominance: Exceeding Parental Traits

Related to heterozygote advantage, overdominance refers to instances where the heterozygous genotype exhibits a phenotype that is more extreme or advantageous than either homozygous genotype.

• Mechanism: This can occur when the interaction of two different alleles creates a novel function or enhances an existing one beyond the capabilities of either allele alone.

• Examples:

  • Certain Crop Hybrids: In agriculture, hybrid crops are often created by crossing two inbred lines. The resulting hybrids can exhibit overdominance, displaying higher yields or increased disease resistance compared to either parent line.
  • Enzyme Activity: In some cases, the heterozygous combination of two enzyme alleles can result in higher overall enzyme activity or broader substrate specificity than either homozygous combination.

• Genetic Basis: Overdominance maintains genetic variation by ensuring that the heterozygous genotype is consistently favored, preventing the fixation of either homozygous allele.

Conclusion: A Symphony of Evolutionary Forces

Genetic variation is not a static property of populations but a dynamic characteristic shaped by a multitude of evolutionary forces. While selection often acts to reduce variation, disruptive selection, frequency-dependent selection, spatially varying selection, heterozygote advantage, mutation-selection balance, gene flow, fluctuating selection, and sexual selection can all contribute to the maintenance or increase of genetic variation. Understanding the interplay of these forces is crucial for comprehending the evolutionary potential of populations and their ability to adapt to changing environments. These mechanisms highlight the complexity and beauty of evolution, demonstrating how nature orchestrates a delicate balance between adaptation and diversity. The continuous generation and maintenance of genetic variation ensures that populations possess the raw material needed to respond to future challenges and opportunities.