17.03 Natural Selection

1. Explain the concept of exponential growth and provide an example of an organism that exhibits this type of population increase.

Exponential Growth:

• Definition: Exponential growth occurs when a population increases at a constant rate per unit time, resulting in the population size growing rapidly without any constraints. This type of growth is represented by a J-shaped curve on a graph.
• Characteristics:
  • Unlimited Resources: Assumes an abundance of resources such as food, water, and shelter.
  • No Predators or Diseases: Absence of natural threats allows continuous growth.
  • High Reproductive Rate: Organisms reproduce rapidly, leading to swift population increases.

Example: Rabbits

• Reproductive Capacity: Rabbits are known for their high fertility rates. A single pair of rabbits can produce multiple litters each year, with several offspring per litter.
• Population Explosion: Under ideal conditions with ample food and few predators, rabbit populations can grow exponentially, quickly reaching large numbers.

Visual Representation:

• Graph: On a graph plotting population size against time, exponential growth is depicted as a steep, upward-curving J-shape, indicating rapid population increase.

2. Describe the case study of the rabbit population in Australia. What factors contributed to their rapid population growth, and what measures were effective in controlling their numbers?

Case Study: Rabbit Population in Australia

Historical Background:

• Introduction (1859):
  • Initial Population: 12 pairs of British rabbits were introduced to Australia.
  • Purpose: Brought in as a food source for settlers and for hunting purposes.

Factors Contributing to Rapid Population Growth:

1. Ample Food Supply:
   • Vegetation: Australia provided abundant vegetation, ensuring rabbits had plenty of food.
2. Few Natural Predators:
   • Predator Absence: Native Australian ecosystems had few natural predators to control rabbit populations.
3. Favorable Climate:
   • Environmental Conditions: Suitable climate and habitat facilitated breeding and survival.
4. High Reproductive Rate:
   • Breeding: Rabbits can reproduce quickly, with multiple litters per year.

Outcome:

• Population Explosion: The rabbit population grew exponentially, leading to severe overgrazing.
• Environmental Impact: Overgrazing threatened grazing resources for sheep, causing economic and ecological issues.

Measures Attempted to Control Rabbit Numbers:

1. Rabbit Drives:
   • Description: Organized mass killings to reduce rabbit populations.
   • Effectiveness: Initially insufficient due to the high reproductive rate of rabbits.
2. Biological Control – Myxomatosis:
   • Introduction of Disease: Myxomatosis, a fatal virus transmitted by fleas and mosquitoes, was introduced.
   • Impact: Significantly reduced rabbit populations, providing a more sustainable control method.

Lessons Learned:

• Importance of Natural Predators: The absence of predators can lead to unchecked population growth.
• Effectiveness of Biological Control: Introducing diseases can effectively control invasive species but must be managed carefully to avoid unintended ecological consequences.

3. Differentiate between biotic and abiotic factors that limit population growth, providing two examples of each.

Limiting Factors on Population Growth:

A. Biotic Factors (Living Components):

• Definition: Biotic factors are living elements in an ecosystem that influence the survival and reproduction of organisms.

Examples:

1. Predation:
   • Description: Natural predators (e.g., foxes, stoats) feed on prey species (e.g., rabbits), controlling their population sizes.
   • Impact: Reduces prey population through increased mortality.
2. Competition for Food:
   • Description: Limited food resources lead to competition among individuals for sustenance.
   • Impact: Reduces the availability of food per individual, leading to lower survival and reproduction rates.

B. Abiotic Factors (Non-Living Components):

• Definition: Abiotic factors are non-living physical and chemical elements in the environment that affect organisms.

Examples:

1. Water Supply:
   • Role: Essential for hydration, metabolic processes, and as a habitat for many organisms.
   • Impact: Scarcity of water can restrict population growth by limiting access to necessary resources.
2. Soil Nutrients:
   • Importance: Influences vegetation growth, which serves as food for herbivores.
   • Impact: Poor soil quality reduces plant biomass, limiting food availability for populations dependent on those plants.

Summary Table:

4. How do predator-prey relationships regulate population sizes? Use the rabbit and fox populations as an example.

Predator-Prey Relationships:

• Definition: Interactions between predators (organisms that hunt) and prey (organisms that are hunted) that influence the population dynamics of both groups.

Regulation of Population Sizes:

1. Population Control:
   • Predators Reduce Prey Numbers: Predators consume prey individuals, directly reducing their population size.
   • Example: Foxes preying on rabbits lower the rabbit population.
2. Dynamic Equilibrium:
   • Responsive Predator Populations: An increase in prey (rabbits) leads to an increase in predator (fox) populations due to more available food.
   • Subsequent Prey Reduction: As predator populations grow, they exert more pressure on the prey population, leading to a decrease in prey numbers.
3. Natural Balance:
   • Cycle Maintenance: This cyclical relationship helps maintain a balance, preventing either population from becoming too large or too small.

Example: Rabbit and Fox Populations

• Initial Condition: High rabbit population provides ample food for foxes.
• Effect on Foxes: Increased food availability supports a larger fox population.
• Feedback Loop:
  1. Rabbit Population Increases: Due to high reproductive rates and ample resources.
  2. Fox Population Increases: More rabbits available as food lead to higher fox survival and reproduction rates.
  3. Rabbit Population Decreases: Increased predation by a larger fox population reduces rabbit numbers.
  4. Fox Population Decreases: With fewer rabbits available, the fox population may decline due to reduced food sources.
• Result: This interaction creates a natural check on both populations, maintaining ecosystem stability.

Visual Representation:

• Lotka-Volterra Model: A mathematical model illustrating the cyclical dynamics of predator and prey populations.

5. Why is it impossible for cells to store large amounts of ATP, and how does this relate to the concept of continuous synthesis?

ATP Storage in Cells:

• Limited Storage Capacity:
  • High Turnover Rate: ATP is used rapidly in cellular processes, such as active transport, muscle contraction, and biosynthesis.
  • Energy Demand: Cells require a constant supply of ATP to meet their immediate energy needs.

Reasons Cells Cannot Store Large Amounts of ATP:

1. High Energy Density: ATP molecules contain high-energy phosphate bonds that release energy upon hydrolysis. Storing large amounts would lead to excessive energy release, which can be harmful.
2. Chemical Instability: ATP is not chemically stable over long periods. It tends to hydrolyze quickly unless stabilized by enzymes.
3. Space Constraints: Large amounts of ATP would occupy significant cellular space, which is not feasible given the compact nature of cells.

Continuous Synthesis:

• Necessity of Continuous Production:
  • Immediate Availability: Since ATP cannot be stored in large quantities, cells must synthesize ATP continuously to replenish what is consumed.
  • Dynamic Demand: Cellular activities fluctuate, requiring a steady and adaptable supply of ATP to meet varying energy demands.

Relation to ATP Production Mechanisms:

1. Cellular Respiration:
   • Processes Involved: Glycolysis, Krebs Cycle, Electron Transport Chain.
   • Continuous Production: These pathways constantly generate ATP to supply the cell with energy.
2. Regeneration Systems:
   • Phosphocreatine System (in muscle cells): Quickly regenerates ATP during short bursts of intense activity.
   • Anaerobic Respiration: Provides ATP when oxygen supply is limited.

Summary:

• Inability to Store Large ATP Quantities: Due to high turnover, energy density, and chemical instability.
• Continuous Synthesis Required: Ensures that cells maintain adequate ATP levels to support ongoing and immediate energy needs.

6. Discuss how diseases can act as a limiting factor in population growth. Include an example in your explanation.

Diseases as Limiting Factors:

• Definition: Diseases caused by pathogens (bacteria, viruses, fungi, parasites) can significantly impact population sizes by increasing mortality rates and reducing reproductive success.

Mechanisms of Population Regulation by Diseases:

1. Increased Mortality Rates:
   • Effect: Higher death rates reduce the overall population size.
2. Decreased Reproductive Success:
   • Effect: Infected individuals may have reduced fertility or may die before reproducing.
3. Population Density Dependency:
   • Transmission Rate: Diseases often spread more easily in densely populated areas, where individuals are in close contact.

Example: Myxomatosis in Rabbits

• Background:
  • Introduction: Myxomatosis was introduced in Australia as a biological control method to reduce the overpopulated rabbit species.
  • Pathogen: A virus transmitted by fleas and mosquitoes.

Impact on Rabbit Population:

1. High Mortality Rate:
   • Outcome: Myxomatosis proved highly effective in killing rabbits, drastically reducing their numbers.
2. Reduced Reproduction:
   • Outcome: Surviving rabbits had fewer offspring, limiting population recovery.
3. Population Stabilization:
   • Long-Term Effect: Disease-induced mortality helped stabilize the rabbit population, preventing overgrazing and ecological imbalance.

Additional Considerations:

• Evolutionary Pressure: Diseases can drive evolutionary changes, such as the development of resistance in populations.
• Ecosystem Impact: Reduction in one species due to disease can have cascading effects on the ecosystem, affecting predators, prey, and vegetation.

Conclusion:

• Role in Population Control: Diseases serve as natural checks on population sizes, preventing unchecked growth and maintaining ecological balance.
• Management Implications: While diseases can be effective in controlling invasive species, careful management is necessary to avoid unintended ecological consequences.

7. Explain the logistic growth model and how it differs from the exponential growth model. What role do limiting factors play in this model?

Logistic Growth Model:

Definition:

• The logistic growth model describes how a population grows rapidly at first but then slows as it approaches the carrying capacity of its environment, resulting in an S-shaped (sigmoidal) curve.

Key Characteristics:

1. Initial Phase – Exponential Growth:
   • Description: When the population is small, resources are abundant, allowing for rapid growth similar to exponential growth.
2. Growth Slowing Phase:
   • Description: As the population size increases, resources become limited, leading to competition, increased predation, and higher mortality rates.
3. Carrying Capacity (K):
   • Definition: The maximum population size that an environment can sustainably support.
   • Equilibrium: The population stabilizes around the carrying capacity, maintaining a balance between birth and death rates.

Differences from Exponential Growth Model:

• Exponential Growth:
  • Curve Shape: J-shaped.
  • Assumptions: Unlimited resources, no predators, no limiting factors.
  • Outcome: Continuous, unchecked population increase.
• Logistic Growth:
  • Curve Shape: S-shaped.
  • Assumptions: Presence of limiting factors, finite resources.
  • Outcome: Population growth slows and stabilizes as it approaches carrying capacity.

Role of Limiting Factors in Logistic Growth:

• Regulation Mechanism: Limiting factors such as food scarcity, predation, disease, and competition become more influential as the population size increases.
• Impact on Growth Rate:
  • Negative Feedback: As the population grows, limiting factors intensify, reducing the growth rate.
  • Population Stabilization: These factors prevent the population from exceeding the carrying capacity, ensuring long-term sustainability.

Graphical Representation:

• S-Shaped Curve: Illustrates how population growth starts exponentially but levels off as it nears the carrying capacity due to the increasing influence of limiting factors.

Real-World Example:

• Rabbit Population in Australia:
  • Initial Rapid Growth: After introduction, rabbits experienced exponential growth due to ample resources and few predators.
  • Slowing Growth: Over time, factors like predation, disease (myxomatosis), and resource depletion limited further growth.
  • Stabilization: Population eventually stabilized near the carrying capacity of the Australian environment.