krit.club logo

Organisms and Populations - Life History Variation

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Organisms evolve to maximize their reproductive fitness, also known as Darwinian fitness, which is represented by a high rr value in the habitat they occupy.

•

Selection pressures in a habitat lead organisms to evolve towards the most efficient reproductive strategy possible.

•

Breeding Frequency Variation: Some species breed only once in their lifetime (semelparity), such as the Pacific Salmon fish and Bamboo. Others breed multiple times during their lifetime (iteroparity), such as most birds and mammals.

•

Offspring Strategy Variation: There is a trade-off between the size and number of offspring. Some organisms produce many small-sized offspring (e.g., Oysters, pelagic fishes), whereas others produce a few large-sized offspring (e.g., birds, mammals).

•

Ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by both the abiotic (non-living) and biotic (living) components of their habitat.

•

The intrinsic rate of natural increase (rr) is a critical parameter for assessing the impact of any biotic or abiotic factor on population growth.

📐Formulae

r=b−dr = b - d

dNdt=rN\frac{dN}{dt} = rN

Nt=N0ertN_t = N_0 e^{rt}

💡Examples

Problem 1:

In a specific habitat, a population of birds has a per capita birth rate bb of 0.80.8 and a per capita death rate dd of 0.30.3. If the initial population size NN is 500500, calculate the intrinsic rate of natural increase rr and the rate of change in population size dNdt\frac{dN}{dt}.

Solution:

First, we calculate the intrinsic rate of natural increase rr: r=b−dr = b - d r=0.8−0.3=0.5r = 0.8 - 0.3 = 0.5

Next, we calculate the rate of change in population size: dNdt=r×N\frac{dN}{dt} = r \times N dNdt=0.5×500=250\frac{dN}{dt} = 0.5 \times 500 = 250

Explanation:

The value r=0.5r = 0.5 represents the Darwinian fitness of the bird population under current environmental conditions. A positive dNdt\frac{dN}{dt} of 250250 indicates that the population is growing by 250250 individuals per unit of time.

Problem 2:

Contrast the reproductive strategies of Oysters and Mammals based on offspring characteristics.

Solution:

Oysters follow a strategy of producing a large number of offspring (n→∞n \to \infty) but each offspring has a very small size (s→0s \to 0). Conversely, Mammals produce a small number of offspring (n→0n \to 0) but each offspring has a large size (s→∞s \to \infty).

Explanation:

This reflects an evolutionary trade-off. Oysters invest less energy per individual offspring, relying on sheer numbers for survival of the species, while Mammals invest high energy and parental care into fewer individuals to ensure higher survival rates per offspring.