Causes of Changing Allele Frequencies

Introduction

• Allele frequency: The proportion of a specific allele within a population’s gene pool.
• A change in allele frequencies indicates that evolution is occurring within the population.
• Key factors influencing allele frequency:
  • Environmental selection pressures
  • Genetic drift
  • Gene flow
  • Mutations

KEY TAKEAWAY: Allele frequencies changing over time is the fundamental definition of evolution occurring at the population level.

Environmental Selection Pressures and Natural Selection

• Environmental selection pressures: External factors that affect an organism’s ability to survive and reproduce. These can be:

  • Physical: Temperature, rainfall, sunlight
  • Biological: Competition for resources, predation, disease
  • Chemical: Pollutants, toxins, pH levels

• Natural selection: The process where individuals with traits better suited to their environment survive and reproduce more successfully, passing on those advantageous traits to their offspring.

  • Leads to a change in allele frequencies as advantageous alleles become more common.
  • Acts on the phenotype, but results in changes in the genotype (allele frequencies).
• Fitness: A measure of an organism’s ability to survive and reproduce in a particular environment. Individuals with higher fitness are more likely to contribute their alleles to the next generation.
• Examples:
  • Antibiotic resistance in bacteria: Bacteria with genes conferring resistance to antibiotics survive and reproduce, increasing the frequency of resistance alleles.
  • Peppered moths during the Industrial Revolution: Dark-colored moths became more common in polluted areas due to better camouflage, increasing the frequency of the dark allele.

EXAM TIP: When describing natural selection, always mention the selective pressure, the variation in the population, the survival and reproduction of the fittest individuals, and the change in allele frequency over time.

Genetic Drift

• Genetic drift: Random fluctuations in allele frequencies due to chance events.
  • Has a greater impact on small populations.
  • Can lead to the loss of alleles, even beneficial ones, or the fixation of harmful alleles.
  • Reduces genetic diversity.
• Two main types of genetic drift:
  • Bottleneck effect: A drastic reduction in population size due to a chance event (e.g., natural disaster). The surviving population has a different allele frequency than the original population.
    • Example: A forest fire kills most of a population of plants. The surviving plants may not represent the original genetic diversity.
  • Founder effect: A small group of individuals colonizes a new area. The founding population has a different allele frequency than the original population.
    • Example: A few birds migrate to a new island. The allele frequencies on the island may be different from the mainland population.

COMMON MISTAKE: Students often confuse natural selection and genetic drift. Remember, natural selection is driven by environmental pressures and fitness, while genetic drift is driven by random chance.

Gene Flow

• Gene flow: The transfer of alleles between populations due to the migration of individuals or the movement of gametes (e.g., pollen).
• Increases genetic diversity within a population.
• Reduces genetic differences between populations.
• Can introduce new alleles or change the frequency of existing alleles.
• Example:
  • Pollen from one population of plants is carried by wind to another population, introducing new alleles.
  • Immigration of individuals from one country to another, introducing new genes to the existing population.

STUDY HINT: Create diagrams to visualize gene flow between populations. This will help you understand how migration can change allele frequencies.

Mutations

• Mutation: A change in the DNA sequence.
• The ultimate source of new alleles.
• Can be:
  • Harmful: Decrease fitness
  • Neutral: No effect on fitness
  • Beneficial: Increase fitness
• Mutations occur randomly, but selection acts on the variation created by mutations.
• Mutation rate is generally low, so mutations alone usually don’t cause large changes in allele frequencies. However, mutation provides the raw material for evolution by creating new alleles.
• Example:
  • A mutation in a gene for hemoglobin can cause sickle cell anemia. If a mutation confers resistance to a disease, it can be beneficial.

REMEMBER: Mutations are random, but natural selection is not. Mutations create the variation, and selection determines which variants become more common.

Summary Table

APPLICATION: Understanding these factors is crucial for conservation efforts. For example, maintaining gene flow between fragmented populations can help preserve genetic diversity.

VCAA FOCUS: VCAA exams often present scenarios where you need to identify the factors that are influencing allele frequencies in a population. Be prepared to analyze data and apply your knowledge to real-world examples.