17.04 Selection Pressures and Survival

1. Survival and Reproduction

• Survival: Not all individuals in a population survive to adulthood. Survival can be influenced by random events or by specific adaptive traits that enhance an organism’s ability to live in its environment.
• Reproduction: Successful reproduction ensures that an organism’s genes are passed on to the next generation. Traits that improve reproductive success become more common over time.
• Fitness:
  • Definition: Fitness refers to an organism’s ability to survive and reproduce, thereby transmitting its alleles (gene variants) to offspring.
  • Importance: High fitness increases the likelihood that an organism’s genes will be represented in future generations.

2. Example: Coat Color in Rabbits

• Genetic Basis:
  • Agouti (Brown) Fur: Dominant allele (A). Provides effective camouflage in natural habitats, reducing the risk of predation.
  • White Fur: Recessive allele (a). Lacks effective camouflage, making individuals more conspicuous to predators.
• Selection Dynamics:
  • Camouflage Advantage: Brown rabbits blend into their environment, enhancing survival and increasing their chances of reproducing.
  • Predation Risk: White rabbits are more easily spotted by predators, decreasing their survival and reproductive success.
• Evolutionary Outcome:
  • Over time, the frequency of the agouti (A) allele increases in the population.
  • The frequency of the white (a) allele decreases and may eventually vanish if selective pressures remain consistent.

3. Impact of Predation as a Selection Pressure

• Definition: Predation is a natural selection pressure where predators influence the survival and reproductive success of prey based on specific traits.
• Mechanism:
  • Traits that reduce the likelihood of being eaten (e.g., camouflage, speed) are favored.
  • Traits that increase visibility to predators are selected against.
• Long-Term Effects:
  • Enhanced traits become more prevalent within the population.
  • Less advantageous traits diminish in frequency, potentially disappearing.

4. Types of Selection in Populations

• Natural selection can manifest in various forms, each shaping populations differently:

a. Stabilizing Selection

• Definition: Stabilizing selection maintains the status quo by favoring average phenotypes and reducing variation.
• Effect on Population:
  • Allele Frequencies: Remain relatively constant.
  • Genetic Diversity: Decreases as extreme phenotypes are selected against.
• Example: Agouti Rabbits
  • Optimal Trait: Brown fur provides the best camouflage.
  • Outcome: Selection maintains the dominant brown coat color, preventing extreme variations like unusually light or dark fur.

b. Directional Selection

• Definition: Directional selection favors one extreme phenotype over others, causing a shift in allele frequencies in one direction.
• Effect on Population:
  • Allele Frequencies: Shift towards the favored extreme.
  • Genetic Diversity: Can increase if the environment continues to favor the extreme trait.
• Examples:
  • Rabbits in Snowy Conditions:
    • Scenario: Increased snowfall leads to white environments.
    • Selection Pressure: White rabbits gain camouflage advantage.
    • Outcome: Frequency of the white (a) allele increases.
  • Industrial Melanism in the Peppered Moth (Biston betularia):
    • Pre-Industrial Era:
      • Dominant Form: Speckled (c) moths blend with lichen-covered trees.
      • Selection Pressure: Natural predation favors speckled moths in unpolluted areas.
    • Industrial Era:
      • Environmental Change: Pollution darkens tree bark by killing lichens.
      • Favored Form: Black (C) moths blend with darkened trees.
      • Outcome: Frequency of the black (C) allele increases near industrial areas.
    • Post-1970s Environmental Reversal:
      • Pollution Control: Reduction in pollution allows lichens to regrow.
      • Selection Pressure Reversed: Speckled moths regain camouflage advantage.
      • Outcome: Frequency of the speckled (c) allele increases.

c. Disruptive Selection

• Definition: Disruptive selection favors both extreme phenotypes over intermediate forms, promoting diversity within a population.
• Effect on Population:
  • Allele Frequencies: Increase for alleles associated with extreme traits.
  • Genetic Diversity: Increases as multiple phenotypes are maintained.
• Example:
  • Body Size in a Population:
    • Scenario: Both very large and very small body sizes confer survival advantages in different niches.
    • Outcome: Both large and small individuals thrive, while intermediate sizes become less common.

5. Mutation and Selection

• Mutation:
  • Definition: Random changes in the DNA sequence that can create new alleles.
  • Role in Evolution: Provides the genetic variation upon which natural selection acts.
• Interaction with Selection:
  • Mutation Rate: The rate at which new alleles (e.g., the black (C) allele in moths) appear is generally constant and not influenced by environmental factors.
  • Selection Pressure: Determines which alleles increase or decrease in frequency based on their impact on fitness.

6. Directional Selection and Antibiotic Resistance in Bacteria

• Antibiotic Resistance:
  • Mechanism:
    • Resistance genes often reside on plasmids, which can be transferred between bacteria through horizontal gene transfer.
    • Random mutations can also confer resistance (e.g., MRSA producing penicillinase).
  • Selection Pressure:
    • Presence of Antibiotics: Environments with antibiotics kill susceptible bacteria.
    • Survival of Resistant Strains: Bacteria with resistance genes survive and reproduce, spreading the resistance alleles rapidly.
• Example: Methicillin-Resistant Staphylococcus aureus (MRSA):
  • Mutation: Development of penicillinase enzyme that deactivates methicillin.
  • Spread: Resistant bacteria proliferate quickly, potentially growing from a single bacterium to billions within 24 hours.
• Human Influence:
  • Overuse of Antibiotics: Increases the selection pressure for resistant strains, accelerating the spread of resistance.
  • Hospital Settings: High antibiotic usage and close patient proximity facilitate the rapid dissemination of resistant bacteria.
• Interventions:
  • Restricting Antibiotic Use: Reducing unnecessary prescriptions helps slow the emergence and spread of resistance.
  • Outcome: Implementation of stricter antibiotic use guidelines post-2007 led to a decline in MRSA cases.

7. Key Terms

• Antibiotic Resistance: The ability of bacteria to survive and proliferate despite the presence of antibiotic drugs, often due to genetic mutations or acquisition of resistance genes.
• Selection Pressure: Environmental factors (e.g., predators, climate, antibiotics) that influence the survival and reproductive success of organisms, thereby affecting allele frequencies in a population.
• Natural Selection: The process by which individuals with favorable traits are more likely to survive and reproduce, leading to an increase in the frequency of those traits in the population over generations.
• Polymorphism: The presence of two or more distinct phenotypes within a population, maintained by factors such as disruptive selection or varying environmental conditions.
• Fitness: An organism’s ability to survive and reproduce, thereby passing its genes to the next generation.
• Allele Frequency: The proportion of a specific allele relative to the total number of alleles for that gene in a population.