Biological Consequences of Changing Allele Frequencies

Introduction to Genetic Diversity

• Genetic diversity refers to the range of different alleles present within a population’s gene pool.
• A population with high genetic diversity possesses a large number of different alleles for each gene.
• A population with low genetic diversity possesses a small number of different alleles for each gene.
• Genetic diversity is crucial for a population’s ability to adapt to changing environmental conditions.

KEY TAKEAWAY: Genetic diversity is the foundation of a population’s adaptability.

Factors Influencing Allele Frequencies

Allele frequencies within a population can change due to various factors:

• Environmental Selection Pressures:
  • Environmental factors that favor certain alleles over others.
  • Can lead to an increase in the frequency of advantageous alleles and a decrease in the frequency of disadvantageous alleles.
  • Example: Antibiotic resistance in bacteria.
• Genetic Drift:
  • Random changes in allele frequencies due to chance events.
  • Has a greater impact on small populations.
  • Examples:
    • Bottleneck effect: A sharp reduction in population size due to a catastrophic event (e.g., natural disaster).
    • Founder effect: A small group of individuals colonizes a new area, carrying only a subset of the original population’s alleles.
• Gene Flow:
  • The movement of alleles between populations.
  • Can introduce new alleles into a population or alter the frequencies of existing alleles.
  • Tends to reduce genetic differences between populations.
• Mutations:
  • The source of new alleles in a population.
  • Can be beneficial, harmful, or neutral.
  • Mutation rate is generally low, but mutations are essential for long-term evolutionary change.

EXAM TIP: Understand how each factor (selection, drift, flow, mutation) impacts the allele frequencies and genetic diversity differently.

Consequences of Increased Genetic Diversity

• Increased adaptability:
  • A population with high genetic diversity is more likely to contain individuals with alleles that are advantageous in a changing environment.
  • This increases the population’s ability to survive and reproduce under new conditions.
• Reduced risk of extinction:
  • High genetic diversity provides a buffer against environmental changes and diseases.
  • A population with low genetic diversity is more vulnerable to extinction if it faces a new challenge.
• Increased evolutionary potential:
  • High genetic diversity provides the raw material for natural selection to act upon.
  • This allows the population to evolve and adapt to new environments over time.

APPLICATION: Crop breeding programs often aim to increase genetic diversity to improve disease resistance and yield.

Consequences of Decreased Genetic Diversity

• Decreased adaptability:
  • A population with low genetic diversity has a limited ability to adapt to changing environmental conditions.
  • This makes the population more vulnerable to extinction.
• Increased risk of disease:
  • Low genetic diversity can make a population more susceptible to disease outbreaks.
  • If all individuals are genetically similar, a single disease can wipe out the entire population.
• Increased risk of inbreeding depression:
  • In small populations with low genetic diversity, inbreeding (mating between closely related individuals) is more likely to occur.
  • Inbreeding can lead to an increased frequency of harmful recessive alleles, which can reduce fitness.
• Reduced evolutionary potential:
  • Low genetic diversity limits the ability of natural selection to act, preventing the population from adapting to new environments.

COMMON MISTAKE: Confusing genetic drift with natural selection. Remember, genetic drift is random, while natural selection is driven by environmental pressures.

Table: Comparing High and Low Genetic Diversity

Examples

• Cheetahs: Cheetahs have very low genetic diversity due to a past bottleneck event. This makes them highly vulnerable to disease and environmental changes.
• Tasmanian Devils: Tasmanian devils have low genetic diversity and are threatened by Devil Facial Tumour Disease (DFTD), a contagious cancer.
• Island Populations: Island populations often have lower genetic diversity due to the founder effect and limited gene flow.

VCAA FOCUS: Be prepared to discuss specific examples of populations with high and low genetic diversity, and the consequences they face.

Manipulation of Gene Pools

• Selective breeding: Humans intentionally select individuals with desirable traits to breed, leading to changes in allele frequencies and reduced genetic diversity in domesticated species.
  • Example: Breeding dogs for specific traits.

STUDY HINT: Create flashcards with examples of how different factors affect genetic diversity.

Bacterial Resistance and Viral Antigenic Variation

• Bacterial resistance:
  • The ability of bacteria to resist the effects of antibiotics.
  • Driven by natural selection: bacteria with resistance genes survive and reproduce in the presence of antibiotics.
  • Leads to an increase in the frequency of resistance alleles and a decrease in the effectiveness of antibiotics.
• Viral antigenic drift:
  • Small, gradual changes in the viral genome due to mutations.
  • Allows the virus to evade the host’s immune system.
• Viral antigenic shift:
  • A sudden, major change in the viral genome due to reassortment of genetic material.
  • Can lead to the emergence of new viral strains that are highly infectious and deadly.

REMEMBER: Antigenic drift is gradual, while antigenic shift is a sudden, major change.

Ongoing Challenges for Treatment and Vaccination

• Bacterial resistance and viral antigenic variation pose ongoing challenges for treatment strategies and vaccination.
• New antibiotics and vaccines must be developed to keep pace with the evolution of pathogens.
• Strategies to slow the evolution of resistance and antigenic variation include:
  • Using antibiotics and antiviral drugs judiciously.
  • Developing vaccines that target multiple viral strains.
  • Improving hygiene and sanitation practices.

KEY TAKEAWAY: The constant evolution of pathogens necessitates ongoing research and development of new treatments and preventative measures.