Does Natural Selection Increase Genetic Variation

Natural selection, a cornerstone of evolutionary biology, often brings to mind the survival of the fittest and the reduction of unfavorable traits. However, the relationship between natural selection and genetic variation is more nuanced than a simple increase or decrease. Natural selection's influence on genetic variation is complex, context-dependent, and pivotal for the long-term adaptability of populations.

Understanding Genetic Variation

Genetic variation refers to the differences in genes and DNA sequences among individuals within a population. This variation is the raw material upon which natural selection acts. Sources of genetic variation include:

• Mutations: Random changes in DNA sequences that can introduce new alleles (versions of a gene).
• Gene Flow: The movement of genes between populations, which can introduce new alleles or change allele frequencies.
• Sexual Reproduction: The shuffling of genes during meiosis and fertilization, creating new combinations of alleles.
• Genetic Drift: Random changes in allele frequencies, particularly in small populations, which can lead to the loss of some alleles and the fixation of others.

Genetic variation is essential for a population's ability to adapt to changing environments. Without it, a population is less likely to have individuals with traits that allow them to survive and reproduce under new conditions.

The Basics of Natural Selection

Natural selection is the process by which certain traits become more or less common in a population over time, based on their impact on survival and reproduction. The key principles of natural selection are:

• Variation: Individuals within a population vary in their traits.
• Inheritance: Traits can be passed from parents to offspring.
• Differential Survival and Reproduction: Individuals with certain traits are more likely to survive and reproduce than others.
• Adaptation: Over time, the population becomes better adapted to its environment as advantageous traits become more common.

Natural selection can take different forms, including:

• Directional Selection: Favors one extreme phenotype, causing a shift in the population's genetic variance toward that extreme.
• Stabilizing Selection: Favors intermediate phenotypes, reducing genetic variance by selecting against extreme traits.
• Disruptive Selection: Favors both extreme phenotypes, potentially increasing genetic variance by selecting against intermediate traits.

Does Natural Selection Increase Genetic Variation? A Complex Relationship

The question of whether natural selection increases genetic variation does not have a straightforward answer. Natural selection can both increase and decrease genetic variation, depending on the specific circumstances.

Scenarios Where Natural Selection Decreases Genetic Variation

1. Stabilizing Selection:

Stabilizing selection occurs when the environment favors intermediate phenotypes over extreme ones. This type of selection reduces genetic variation by selecting against individuals with extreme traits.

• Example: Human birth weight. Babies with very low or very high birth weights have higher mortality rates than babies with intermediate birth weights. As a result, stabilizing selection favors genes that contribute to an average birth weight, reducing the genetic variance for this trait.

2. Directional Selection:

Directional selection occurs when the environment favors one extreme phenotype. While it may initially seem like directional selection could increase genetic variation by promoting certain alleles, it typically leads to a reduction in overall genetic diversity. As the favored allele becomes more common, other alleles may be lost from the population.

• Example: Antibiotic resistance in bacteria. When antibiotics are used, bacteria with resistance genes have a higher survival rate. Over time, the frequency of these resistance genes increases in the population, but other, non-resistance genes may become less common, leading to a decrease in overall genetic variation.

3. Purifying Selection:

Purifying selection, also known as negative selection, is the removal of deleterious (harmful) alleles from a population. This type of selection is essential for maintaining the fitness of a population by preventing the accumulation of harmful mutations. However, it also reduces genetic variation by eliminating these alleles.

• Example: Many genetic disorders are caused by deleterious recessive alleles. Purifying selection acts to remove these alleles from the population, reducing the genetic variance for traits related to these disorders.

Scenarios Where Natural Selection Increases Genetic Variation

1. Disruptive Selection:

Disruptive selection occurs when the environment favors both extreme phenotypes over intermediate ones. This type of selection can increase genetic variation by promoting the survival and reproduction of individuals with diverse traits.

• Example: Beak size in Darwin's finches. On an island with two types of seeds—small and large—finches with either small or large beaks are more successful at obtaining food than finches with intermediate beak sizes. Over time, this can lead to a population with increased genetic variation for beak size.

2. Frequency-Dependent Selection:

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

• Example: Scale-eating fish in Lake Tanganyika. These fish have two morphs—one with mouths that twist to the left and one with mouths that twist to the right. The fish attack their prey (other fish) by sneaking up and biting their scales. The prey fish become wary of attacks from the more common side, giving a selective advantage to the rarer morph. This frequency-dependent selection maintains genetic variation for mouth direction.

3. Heterozygote Advantage:

Heterozygote advantage occurs when individuals with heterozygous genotypes (i.e., carrying two different alleles for a gene) have higher fitness than individuals with homozygous genotypes (i.e., carrying two identical alleles for a gene). This type of selection can maintain genetic variation by preventing the loss of alleles.

• Example: Sickle cell anemia in regions where malaria is prevalent. Individuals who are heterozygous for the sickle cell allele (i.e., carrying one normal allele and one sickle cell allele) are resistant to malaria and do not suffer from severe sickle cell symptoms. This heterozygote advantage maintains the sickle cell allele in the population, even though it is harmful in homozygous individuals.

4. Spatially Varying Selection:

Spatially varying selection occurs when different environments within a population's range favor different traits. This can lead to the maintenance of genetic variation across the population as a whole.

• Example: Plant populations near mine sites. Soils near mines often contain high levels of heavy metals, which are toxic to most plants. However, some plants have evolved resistance to these metals. In areas with contaminated soil, metal-resistant plants are favored, while in areas with uncontaminated soil, non-resistant plants may be favored due to trade-offs associated with resistance. This spatially varying selection maintains genetic variation for metal resistance across the plant population.

5. Sexually Antagonistic Selection:

Sexually antagonistic selection occurs when the same genes have different effects on fitness in males and females. This can lead to the maintenance of genetic variation because alleles that are beneficial in one sex may be harmful in the other.

• Example: Body size in some insects. Larger body size may be advantageous for males in competition for mates, but disadvantageous for females due to increased energy requirements. This sexually antagonistic selection can maintain genetic variation for body size in the population.

The Role of Mutation in Maintaining Genetic Variation

While natural selection can sometimes reduce genetic variation, mutation is a fundamental source of new genetic variation. Mutations introduce new alleles into the population, providing the raw material upon which natural selection can act.

• Mutation-Selection Balance: The balance between the introduction of new alleles by mutation and the removal of deleterious alleles by selection is known as mutation-selection balance. This balance can maintain genetic variation in the population, even when selection is acting to reduce it.
• Adaptive Potential: The presence of genetic variation, even if it is not currently advantageous, provides a population with the potential to adapt to future environmental changes.

Examples of Natural Selection and Genetic Variation in Real Populations

To illustrate the complex relationship between natural selection and genetic variation, let's consider a few real-world examples:

1. Peppered Moths (Biston betularia):

The classic example of industrial melanism in peppered moths demonstrates how natural selection can alter genetic variation in response to environmental change.

• Background: Before the Industrial Revolution, the majority of peppered moths in England were light-colored, providing camouflage against lichen-covered trees. A darker, melanic form was rare.
• Industrial Revolution: As industrial pollution darkened the trees, the light-colored moths became more visible to predators, while the melanic moths were better camouflaged.
• Natural Selection: Natural selection favored the melanic moths, leading to an increase in their frequency in the population. This is an example of directional selection.
• Genetic Variation: While the frequency of the melanic allele increased, the overall genetic variation in the moth population may have decreased as other alleles became less common. However, the presence of multiple alleles for coloration allowed for the rapid adaptation to the changing environment.

2. Darwin's Finches:

Darwin's finches on the Galápagos Islands provide a compelling example of how natural selection can drive diversification and maintain genetic variation in beak size and shape.

• Background: Different islands have different types of seeds available.
• Natural Selection: Finches with beak shapes best suited to the available seeds have a higher survival rate. This has led to the evolution of different beak morphologies on different islands.
• Genetic Variation: Disruptive selection, frequency-dependent selection, and spatially varying selection all contribute to maintaining genetic variation in beak size and shape across the finch populations.

3. Human Lactase Persistence:

The evolution of lactase persistence (the ability to digest lactose as adults) in some human populations is another example of natural selection acting on genetic variation.

• Background: Most mammals lose the ability to digest lactose after infancy because the gene that codes for lactase production is turned off.
• Evolution of Lactase Persistence: In populations with a long history of dairy farming, individuals with mutations that keep the lactase gene turned on into adulthood have a selective advantage because they can utilize milk as a food source.
• Natural Selection: Natural selection favored these mutations, leading to an increase in the frequency of lactase persistence in these populations.
• Genetic Variation: The genetic variation for lactase persistence varies across different human populations, reflecting the different selective pressures they have faced.

Implications for Conservation Biology

Understanding the relationship between natural selection and genetic variation is crucial for conservation biology.

• Maintaining Genetic Diversity: Conservation efforts should focus on maintaining genetic diversity within populations to ensure their ability to adapt to future environmental changes.
• Avoiding Bottlenecks: Population bottlenecks, which occur when a population undergoes a drastic reduction in size, can lead to a loss of genetic variation. Conservation strategies should aim to avoid bottlenecks and promote gene flow between populations to maintain genetic diversity.
• Managing Selection Pressures: Human activities, such as habitat destruction, pollution, and climate change, can alter selection pressures on populations. Conservation managers need to understand these altered selection pressures and take steps to mitigate their negative impacts on genetic diversity.

Conclusion

In conclusion, the relationship between natural selection and genetic variation is complex and multifaceted. Natural selection can both increase and decrease genetic variation, depending on the specific circumstances. While stabilizing selection, directional selection, and purifying selection tend to reduce genetic variation, disruptive selection, frequency-dependent selection, heterozygote advantage, spatially varying selection, and sexually antagonistic selection can maintain or increase genetic variation. Mutation is a fundamental source of new genetic variation, providing the raw material upon which natural selection acts. Understanding this interplay is essential for comprehending the process of evolution and for developing effective conservation strategies. By conserving genetic diversity, we can help ensure that populations have the adaptive potential to thrive in a changing world.