Does Natural Selection Increase Or Decrease Genetic Variation

Does Natural Selection Increase or Decrease Genetic Variation? A Complex Relationship

Natural selection, the cornerstone of Darwinian evolution, is a powerful force shaping life on Earth. It acts upon the existing variation within a population, favoring traits that enhance survival and reproduction. However, the effect of natural selection on genetic variation itself is a nuanced and often counterintuitive relationship. While it can decrease variation in certain scenarios, it can also paradoxically increase it in others, leading to a complex interplay that drives the evolution of species.

The Apparent Paradox: Reducing Variation through Selection

The most straightforward effect of natural selection is the reduction of genetic variation. This occurs when selection strongly favors a particular allele (variant of a gene), leading to its increased frequency within the population. As this favored allele becomes more common, alternative alleles become less common, potentially disappearing altogether. This process is called directional selection.

Examples of Directional Selection Reducing Variation:

• Peppered Moths: The classic example of directional selection involves the peppered moth ( Biston betularia) during the Industrial Revolution. Initially, light-colored moths were dominant, blending in with lichen-covered trees. However, industrial pollution darkened the tree bark, giving a selective advantage to darker moths. Over time, the frequency of the dark allele increased dramatically, significantly reducing the variation in moth coloration.

• Antibiotic Resistance in Bacteria: The overuse of antibiotics has driven a powerful example of directional selection in bacteria. Bacteria with naturally occurring resistance genes are more likely to survive antibiotic treatment. These resistant bacteria reproduce, passing on the resistance genes to their offspring. This results in a population dominated by antibiotic-resistant strains, with a significant loss of genetic variation concerning antibiotic susceptibility.

• Sickle Cell Anemia and Malaria: While seemingly contradictory, the case of sickle cell anemia illustrates how directional selection can reduce variation in a specific gene but potentially maintain overall variation. The sickle cell allele protects against malaria, conferring an advantage in malarial regions. However, this advantage comes at the cost of potential health issues related to sickle cell disease itself. This balancing act maintains both the sickle cell allele and the normal allele within the population, albeit at different frequencies in malarial regions. This is an example of balancing selection, a more nuanced effect of natural selection.

The Unexpected Twist: Mechanisms Increasing Genetic Variation

Despite the common perception of natural selection as a purely homogenizing force, it can also contribute to increasing genetic variation. This happens through several mechanisms:

1. Maintaining Polymorphism Through Balancing Selection:

Balancing selection, as mentioned earlier, maintains multiple alleles within a population. This occurs when heterozygotes (individuals with two different alleles) have a higher fitness than homozygotes (individuals with two identical alleles).

• Heterozygote Advantage: In the case of sickle cell anemia, heterozygotes possess some malaria resistance without suffering from severe sickle cell disease. This advantage maintains both alleles within the population, preserving genetic variation.

• Frequency-Dependent Selection: The fitness of an allele depends on its frequency in the population. Rare alleles might have an advantage simply because they are rare, preventing any single allele from becoming dominant. This is often seen in predator-prey relationships, where rare color morphs can evade predation more effectively.

2. Diversifying Selection (Disruptive Selection):

This type of selection favors the extremes of a trait's distribution while selecting against the intermediate forms. This leads to an increase in the overall variation of that specific trait.

• Darwin's Finches: Different beak sizes in Darwin's finches are a prime example. During times of food scarcity, finches with either very large or very small beaks might be better at accessing different food sources compared to finches with medium-sized beaks. This would lead to an increase in the variation of beak size within the population.

• Sexual Selection: This particular form of natural selection focuses on traits that enhance an organism's ability to obtain mates. Often, extreme traits are favored, leading to increased variation. The extravagant plumage of some bird species is a consequence of sexual selection driving increased variation.

3. Mutation: The Raw Material for Variation:

Natural selection doesn’t create new genetic variation; it acts upon the variation that already exists. Mutations, changes in DNA sequence, are the ultimate source of new genetic variation. While many mutations are deleterious (harmful), some can be beneficial or even neutral. Beneficial mutations provide the raw material for natural selection to act upon, potentially leading to increased variation if those mutations spread within the population.

4. Gene Flow: Introducing New Alleles:

Gene flow, the movement of genes between populations, is another mechanism that can increase genetic variation. When individuals from one population migrate to another and interbreed, they introduce new alleles into the recipient population. This influx of new alleles can enhance the genetic diversity of the recipient population, providing more material for natural selection to act upon.

5. Genetic Recombination: Shuffling Existing Variation:

Sexual reproduction involves the shuffling of existing alleles through recombination during meiosis. This process creates novel combinations of alleles, increasing the phenotypic variation within a population, even without the introduction of new mutations. This process can produce individuals with novel combinations of traits, which then become subject to natural selection.

The Interplay: A Dynamic Equilibrium

The relationship between natural selection and genetic variation is not a simple one of increase or decrease. It’s a dynamic interplay where both processes are constantly interacting and influencing each other. Natural selection often reduces variation in specific traits under directional selection but can simultaneously increase variation through balancing selection, diversifying selection, or the interplay with other evolutionary mechanisms like mutation and gene flow.

The overall effect on genetic variation depends on the specific circumstances: the intensity of selection, the nature of the selective pressures, the rate of mutation, the level of gene flow, and the population size. In smaller populations, genetic drift (random fluctuations in allele frequencies) can play a significant role, potentially leading to reduced variation even without strong natural selection.

Conclusion: A Complex Dance of Evolution

In conclusion, the relationship between natural selection and genetic variation is a multifaceted and dynamic process. While natural selection frequently reduces genetic variation in specific traits, it can paradoxically increase overall genetic variation through various mechanisms like balancing selection, diversifying selection, and by interacting with the constant influx of new variation from mutation and gene flow. Understanding this complex interplay is crucial to comprehending the evolution of life's incredible diversity. The outcome is not a simple reduction of variation, but a complex dance of evolutionary forces shaping the genetic landscape of populations and driving the ongoing process of adaptation and diversification. The ongoing research into this complex interplay continues to refine our understanding of the evolutionary process and the remarkable diversity of life on Earth.