Does Natural Selection Decrease Genetic Variation

Natural selection, a cornerstone of evolutionary biology, is the process by which certain traits become more or less common in a population due to differences in survival and reproductive success. The interplay between natural selection and genetic variation is a complex one, and the question of whether natural selection decreases genetic variation requires a nuanced understanding of various evolutionary mechanisms. This article delves into the intricacies of this relationship, exploring how natural selection can both decrease and, paradoxically, maintain genetic variation.

Introduction: Genetic Variation as the Raw Material for Evolution

Genetic variation is the diversity of genes within a population. It is the raw material upon which natural selection acts. Without genetic variation, there would be no differences in traits for selection to favor or disfavor, and evolution would grind to a halt. Sources of genetic variation include:

• Mutations: Random changes in DNA sequence.
• Gene Flow: The movement of genes between populations.
• Sexual Reproduction: The reshuffling of genes during the formation of sperm and egg, leading to new combinations of traits.
• Genetic Drift: Random fluctuations in allele frequencies, especially in small populations.

Natural Selection: The Sculptor of Populations

Natural selection acts on this existing genetic variation, favoring individuals with traits that enhance their survival and reproduction in a specific environment. This process leads to changes in the frequency of alleles (different versions of a gene) within a population over time. There are several modes of natural selection:

• Directional Selection: Favors one extreme phenotype, causing a shift in the population's genetic variance towards that extreme.
• Stabilizing Selection: Favors intermediate phenotypes, reducing variation around the mean.
• Disruptive Selection: Favors both extreme phenotypes, leading to a bimodal distribution and potentially increasing genetic variation in the short term.
• Balancing Selection: Maintains multiple alleles in a population, preventing any single allele from becoming fixed.

How Natural Selection Decreases Genetic Variation

In many scenarios, natural selection does indeed decrease genetic variation. This occurs primarily through:

1. Eliminating Deleterious Alleles

Natural selection efficiently removes harmful mutations from the population. These deleterious alleles often result in reduced fitness (survival and reproductive success). As selection acts against individuals carrying these alleles, their frequency decreases over generations, eventually leading to their elimination.

Example: Consider a population of moths where a new mutation arises that results in a bright, easily visible color to predators. Moths with this mutation are more likely to be eaten, reducing their chances of passing on the gene to the next generation. Over time, the frequency of this allele will decrease, reducing genetic variation in coloration within the population.

2. Directional Selection and the Fixation of Beneficial Alleles

When a new beneficial mutation arises, directional selection can drive it to fixation, meaning it becomes the only allele present for that gene in the population. As the beneficial allele increases in frequency, other alleles at that locus become less common and may eventually disappear. This process reduces genetic variation at that specific gene.

Example: Antibiotic resistance in bacteria. When antibiotics are introduced into an environment, bacteria with genes that confer resistance have a survival advantage. These resistant bacteria proliferate, and the resistance genes become more common in the population, often leading to the near-elimination of non-resistant strains. This process dramatically reduces genetic variation related to antibiotic resistance.

3. Stabilizing Selection and the Reduction of Extreme Phenotypes

Stabilizing selection favors individuals with intermediate traits, effectively narrowing the range of phenotypic variation in a population. By selecting against extreme phenotypes, it reduces the frequency of alleles that contribute to those extremes, thereby decreasing genetic variation.

Example: Human birth weight. Babies with extremely low or extremely high birth weights are more likely to experience health complications and have lower survival rates. Stabilizing selection favors babies with intermediate birth weights, reducing the variation in birth weight within the population over time.

4. Selective Sweeps

A selective sweep occurs when a strongly beneficial allele arises and rapidly increases in frequency, dragging along with it nearby neutral or even slightly deleterious alleles on the same chromosome. This "hitchhiking" effect reduces genetic variation in the region surrounding the selected gene. The region affected by the sweep shows reduced diversity compared to other parts of the genome.

Example: Lactase persistence in human populations. The ability to digest lactose into adulthood is a relatively recent adaptation in some human populations with a history of dairy farming. The selection for lactase persistence led to a selective sweep, reducing genetic variation in the region around the lactase gene (LCT).

How Natural Selection Maintains Genetic Variation

While natural selection can decrease genetic variation, it can also act to maintain it in several ways:

1. Balancing Selection

Balancing selection refers to several mechanisms that maintain multiple alleles at a locus, preventing any single allele from becoming fixed. This can occur through:

• Heterozygote Advantage: Heterozygous individuals (carrying two different alleles) have higher fitness than either homozygous individual (carrying two identical alleles).
  • Example: Sickle cell anemia. Individuals heterozygous for the sickle cell allele have increased resistance to malaria, providing a survival advantage in regions where malaria is prevalent. This maintains both the normal and sickle cell alleles in the population, even though the homozygous sickle cell condition is harmful.
• Frequency-Dependent Selection: The fitness of an allele depends on its frequency in the population. Rare alleles may have a selective advantage simply because they are rare.
  • Example: Scale-eating fish in Lake Tanganyika. These fish have two morphs, "left-mouthed" and "right-mouthed," which attack their prey from opposite sides. If one morph becomes too common, the prey will learn to guard against attacks from that side, giving the rarer morph an advantage. This frequency-dependent selection maintains both morphs in the population.
• Spatially Varying Selection: Different environments favor different alleles, leading to the maintenance of genetic variation across a species' range.
  • Example: Plant populations near mine tailings. Plants growing near mine tailings are exposed to high levels of heavy metals. Some plants have evolved tolerance to these metals, while others have not. The presence of both tolerant and non-tolerant plants maintains genetic variation within the population.

2. Mutation-Selection Balance

Mutations constantly introduce new alleles into a population, while selection acts to remove deleterious alleles. The mutation-selection balance is the equilibrium point at which the rate of new mutations equals the rate at which selection removes them. This balance maintains a certain level of genetic variation in the population, even for genes under strong selection.

Example: Many human genetic disorders are caused by recessive alleles. These alleles are constantly being created by mutation, but they are also being removed by selection against homozygous individuals who express the disorder. The mutation-selection balance ensures that these alleles persist in the population at a low frequency.

3. Disruptive Selection

Disruptive selection favors extreme phenotypes, which can lead to increased genetic variation in the short term. While it can potentially lead to the formation of distinct subpopulations or even new species, it maintains variability within the larger population.

Example: Beak size in black-bellied seedcrackers. These birds have two main food sources: hard seeds and soft seeds. Birds with large beaks are better at cracking hard seeds, while birds with small beaks are better at handling soft seeds. Disruptive selection favors both large and small beak sizes, while selecting against intermediate beak sizes, maintaining genetic variation in beak morphology.

4. Gene Flow and Migration

Gene flow, the movement of genes between populations, can introduce new alleles into a population or increase the frequency of existing alleles. This can counteract the effects of selection, which might otherwise reduce genetic variation.

Example: The introduction of pesticide resistance genes into a susceptible insect population. If a population of insects is exposed to pesticides, those with resistance genes will have a survival advantage. Gene flow from other populations that already possess resistance genes can introduce these genes into the susceptible population, increasing genetic variation related to pesticide resistance.

The Role of Environmental Variation

The environment plays a crucial role in shaping the relationship between natural selection and genetic variation. A constantly changing environment can prevent any single allele from becoming fixed, as different alleles may be favored at different times or in different places. This fluctuating selection can maintain genetic variation over long periods.

Example: Coloration in peppered moths. During the Industrial Revolution in England, pollution darkened tree bark, favoring dark-colored moths over light-colored moths. As pollution levels decreased, the tree bark became lighter again, and light-colored moths regained their advantage. This fluctuating selection maintained genetic variation in moth coloration.

The Importance of Neutral Variation

Not all genetic variation is subject to natural selection. Neutral variation refers to differences in DNA sequence that do not affect an individual's fitness. These neutral variants can accumulate in a population over time due to genetic drift, without being directly influenced by selection. While neutral variation may not directly contribute to adaptation, it provides a reservoir of genetic diversity that can be acted upon by selection if environmental conditions change.

Example: Synonymous mutations in coding regions. These mutations change the DNA sequence but do not alter the amino acid sequence of the protein. They are often considered neutral because they do not affect protein function. However, they can still play a role in gene regulation or other cellular processes, and they can become subject to selection if the environment changes.

Case Studies: Examples in Real Populations

• Darwin's Finches: The famous finches of the Galapagos Islands provide a classic example of how natural selection can both reduce and maintain genetic variation. Different beak shapes have evolved in response to different food sources, leading to adaptive radiation. While selection favors specific beak shapes in particular environments, gene flow between islands and fluctuating environmental conditions can maintain genetic variation in beak morphology within the overall finch population.
• Rock Pocket Mice: These mice have evolved different coat colors to match the color of their rocky habitats. Dark-colored mice are favored in dark lava flows, while light-colored mice are favored in light-colored desert areas. This spatially varying selection maintains genetic variation in coat color across the species' range.
• Human Skin Color: Human skin color varies geographically, with darker skin being more common in regions with high levels of UV radiation and lighter skin being more common in regions with low levels of UV radiation. This variation is thought to be driven by selection for vitamin D production in low-UV environments and protection against folate degradation in high-UV environments. Gene flow and cultural practices can also influence the distribution of skin color variation.

Implications for Conservation Biology

Understanding the relationship between natural selection and genetic variation is crucial for conservation biology. Populations with low genetic variation are more vulnerable to environmental changes and disease outbreaks, as they lack the raw material for adaptation. Conservation efforts often focus on maintaining or increasing genetic variation in threatened populations through:

• Protecting Habitat: Maintaining diverse habitats can promote spatially varying selection and maintain genetic variation.
• Managing Gene Flow: Facilitating gene flow between isolated populations can introduce new alleles and increase genetic diversity.
• Avoiding Bottlenecks: Population bottlenecks, which occur when a population undergoes a drastic reduction in size, can lead to a loss of genetic variation. Conservation efforts aim to prevent bottlenecks and promote population growth.

Conclusion: A Balancing Act

The relationship between natural selection and genetic variation is complex and multifaceted. While natural selection often acts to decrease genetic variation by eliminating deleterious alleles, driving beneficial alleles to fixation, and favoring intermediate phenotypes, it can also maintain genetic variation through balancing selection, mutation-selection balance, disruptive selection, and gene flow. The specific outcome depends on the mode of selection, the environmental context, and the genetic architecture of the population.

Understanding this dynamic interplay is essential for comprehending the process of evolution and for addressing challenges in conservation biology and human health. By considering the various factors that influence genetic variation, we can gain a deeper appreciation for the remarkable adaptability of life on Earth.