Population Ecology and Evolutionary Principles

Natural Selection and Evolution

Natural selection is a fundamental mechanism of evolution, explaining how traits become more or less common in a population over generations.

• Requirement 1: Variation – There must be heritable variation in traits among individuals in a population.

• Requirement 2: Differential Reproduction – Some individuals must reproduce more successfully than others due to those traits.

• Definition: Natural selection is the process by which individuals with advantageous traits survive and reproduce at higher rates, leading to evolutionary change.

• Example: In a population of rats, those with a gene for disease resistance may survive and reproduce more, increasing the frequency of that gene.

Human Population Size: Past and Present

Understanding human population growth provides context for ecological and evolutionary studies.

• Current Human Population: As of the early 2020s, the global human population is approximately 8 billion.

• Historical Population (1928): The world population in 1928 was about 2 billion.

• Application: These numbers illustrate the rapid growth of the human population over the last century.

Sex Differences in Mammalian Longevity

In many mammalian species, females tend to live longer than males. This phenomenon is observed across a wide range of taxa.

• Key Point: Females often live longer due to a combination of genetic, physiological, and behavioral factors.

• Possible Explanations:

  • Males may experience higher mortality due to riskier behaviors or competition for mates.

  • Hormonal differences (e.g., estrogen in females may have protective effects).

  • Genetic factors, such as the presence of two X chromosomes in females, may provide greater resilience to certain diseases.

• Example: In humans and many other mammals, average female lifespan exceeds that of males.

Aging

Aging is a biological process affecting all living organisms, characterized by a gradual decline in physiological function.

• Definition: Aging is the progressive accumulation of changes in an organism over time, leading to decreased biological function and increased vulnerability to disease and death.

• Key Features:

  • Cellular damage accumulation

  • Decreased reproductive capacity

  • Increased mortality rate with age

Survivorship Curves

Survivorship curves graphically represent the number of individuals surviving at each age for a given species or group.

• Type I Curve: High parental care, low offspring number, high offspring survival (e.g., humans, elephants).

• Type II Curve: Constant mortality rate throughout life (e.g., some birds, reptiles).

• Type III Curve: Low parental care, high offspring number, low early survival (e.g., many fish, plants).

• Type III: Exhibits very little parental care and high numbers of offspring.

• Type I: Exhibits high offspring survival and high parental care.

Population Growth Models

Population growth can be modeled mathematically to predict changes in population size over time.

Exponential Growth

Exponential growth occurs when resources are unlimited, and the population increases at a constant rate per individual.

• Equation:

• Nt: Population size at time t

• N0: Initial population size

• rmax: Maximum per capita growth rate

• t: Time (number of generations)

• When does it occur? Exponential growth occurs when resources are abundant and there are no limiting factors (e.g., after colonization of a new habitat).

• Example Calculation: If at generation 0, N = 2 and rmax = 2, then:

Generation 0: N = 2 Generation 1: N = 2 × 2 = 4 Generation 2: N = 4 × 2 = 8 Generation 3: N = 8 × 2 = 16

Logistic Growth

Logistic growth describes population expansion that decreases as resources become limited, resulting in a population size that stabilizes at the carrying capacity (K).

• Equation:

• Nt: Population size at time t

• rmax: Maximum per capita growth rate

• K: Carrying capacity (maximum population size the environment can support)

• Example Calculation: At generation 0, N = 2, rmax = 2, K = 10:

Generation 0: N = 2 Generation 1: N = 2 + 2 × 2 × (1 - 2/10) = 2 + 4 × 0.8 = 2 + 3.2 = 5.2 Generation 2: N = 5.2 + 2 × 5.2 × (1 - 5.2/10) = 5.2 + 10.4 × 0.48 = 5.2 + 4.992 = 10.192 Generation 3: N = 10.192 + 2 × 10.192 × (1 - 10.192/10) = 10.192 + 20.384 × (-0.0192) ≈ 10.192 - 0.392 ≈ 9.8

• Key Point: As N approaches K, the growth rate slows and the population stabilizes.

Additional info: Calculations for logistic growth are shown stepwise for clarity. Real populations may fluctuate around K due to environmental variability.