Lecture 6

Overview of Populations, Epidemics and Natural Selection

The lecture discusses critical factors affecting survival in populations, including nutrition, predation, and disease. It will examine population dynamics specifically focusing on the interactions between hare and lynx in Canadian forests, using observations leading to hypotheses and experimental verification to explore natural selection.

Population Dynamics of Hare and Lynx

  • The populations of snowshoe hares (Lepus americanus) and Canadian lynxes (Lynx canadensis) exhibit oscillations roughly every decade.

  • Historical data from fur trading (1844-1944) provides a basis for analyzing these fluctuations.

  • The cyclical nature of hare and lynx populations raises questions regarding the underlying causes of these cycles.

Observations Leading to Hypothesis

  • Foundational observation: Fluctuating population densities of hares and lynxes can be tracked, revealing a predictable ten-year cycle.

  • Hypothesis development: Multiple potential explanations emerge to account for the predator-prey dynamics between the two species.

Theoretical Framework: The Lotka-Volterra Model

  • This model posits several assumptions:

    1. The prey (hare) population has sufficient food.

    2. The predator (lynx) population relies entirely on the size of the prey population for food.

    3. Population change rate is proportional to its size.

    4. Environmental conditions remain unchanged.

    5. Predators have a limitless appetite.

  • It offers predictions about hare-lynx population dynamics, particularly that the predator cycle lags behind the prey cycle.

Experimental Testing of the Lotka-Volterra Model

  • Research conducted by Krebs and colleagues from 1976-1995 aimed to validate the model assumptions:

    1. Excluding Predators: When predators were left out of the environment, the cyclical population dynamics were abolished, demonstrating the critical role of predation in population regulation.

    2. Adding Food: Supplemental food did not significantly alter hare population size, indicating that food availability is less impactful than predation in this context.

Chronic Stress in Hares

  • Physiological Definition: Stress is measured by blood cortisol levels and can negatively affect reproductive rates and immune functions.

  • Indicators of predator presence (odors, visual cues) contribute to chronic stress in hares, evidencing the indirect effects of predation on prey populations.

    • Experimental metrics showed cortisol concentration spikes in individual female hares correlate with reduced offspring quantity and viability.

Ecosystem Implications of the Hare-Lynx Cycle

  • The interlinked dynamics of hare and lynx populations underscore broader ecological impacts.

  • As a keystone species, changes in hare populations can influence multiple trophic levels within the boreal forest food web.

Epidemiological Models: Application to Human Disease

  • Introduction to disease dynamics was presented by characterizing infectious diseases using models such as:

    • SIR Model: Represents individuals as:

    • S: Susceptible individuals who can contract the disease.

    • I: Infected individuals capable of transmitting the infection.

    • R: Recovered individuals who have immunity or have died.

    • The flow dynamics between these categories help understand the prevalence and spread of diseases, offering a structured approach to epidemiology.

Case Study: COVID-19 Pandemic

  • Prevalence (P): The proportion of the population that is infected at a given time.

  • Reproductive Rate (R): Defined as the average number of secondary cases produced by one infected case. This helps in modeling the spread and control measures of infectious disease outbreaks.

Evolutionary Theory: The Mechanisms of Natural Selection

  • Natural selection stems from four core principles:

    1. Variation: Individuals within a population exhibit differences.

    2. Inheritance: Variations can be passed down to offspring.

    3. Competition: Populations tend to remain constant in size, driving competition for resources.

    4. Mutation: Random genetic mutations introduce new traits that can be subject to selection.

  • Fitness, defined mathematically as the proportional change in abundance of a genotype, illustrates how natural selection operates:

    • n(t+1) = W n(t)
      where fitness (W) reflects the reproductive success of genotypes over generations.

Genetic Variation and Its Role in Natural Selection

  • Phenotype: The observable traits exhibited by an organism based on its genotype.

  • Trait: Specific characteristics influenced by genetic factors.

  • Environmental influences interact with genotype to shape phenotypic traits, highlighting the complexity of evolution.

Conclusions

  1. Population dynamics can reveal both cyclic changes and the ecological interactions underlying these fluctuations.

  2. Observations, mathematical modeling, and experimental testing together lead to an understanding of ecological and evolutionary principles.

  3. Disease dynamics can be modeled similarly, drawing critical parallels between ecology and epidemiology.

  4. Understanding natural selection is imperative for interpreting evolutionary biology, including implications for public health and conservation.