Organisms and Populations — Class 12 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 12 Biology chapter "Organisms and Populations" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 12 Biology chapter "Organisms and Populations" — 8 important questions with detailed answers for CBSE boar…
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NCERT Solutions for Class 12 Biology — Organisms and Populations. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Organisms and Populations — All 8 Questions Solved
Q1. Define population and community.
POPULATION:
A population is a group of individuals of the same species living in the same geographical area at the same time. Members of a population can interbreed and share a common gene pool. Example: All lions in Gir Forest form a population.
COMMUNITY:
A community is a group of different populations of different species living and interacting in the same geographical area at the same time. It consists of all the biotic factors in an ecosystem. Example: All plants, animals, and microorganisms living in a forest form a community.
Q2. What are the characteristics of a population? Explain population density and population distribution.
CHARACTERISTICS OF A POPULATION:
1. GENE POOL:
- All genes present in a population
- Shows genetic variation among individuals
- Foundation for evolution and adaptation
2. GENE FREQUENCY:
- Proportion of each allele in gene pool
- Expressed as percentage or decimal
- Changes over generations
3. BIRTH RATE (NATALITY):
- Number of new individuals born per unit time
- Expressed as number per 1000 individuals per year
- Increases population size
4. DEATH RATE (MORTALITY):
- Number of individuals dying per unit time
- Expressed as number per 1000 individuals per year
- Decreases population size
5. IMMIGRATION:
- Movement of individuals into a population
- Brings new genes into population
- Increases population size
6. EMIGRATION:
- Movement of individuals out of a population
- Removes genes from population
- Decreases population size
POPULATION DENSITY:
DEFINITION:
- Number of individuals per unit area or unit volume
- Expressed as individuals/m², individuals/hectare, or individuals/km²
CALCULATION:
- Population density = Total population / Total area
- Example: If 1000 deer in 10 km² forest, density = 100 deer/km²
MEASUREMENT METHODS:
1. ABSOLUTE DENSITY:
- Actual number of individuals per unit area
- Obtained by direct counting
- Most accurate method
- Used for large organisms like elephants
2. RELATIVE DENSITY:
- Proportion of a species compared to total population
- Expressed as percentage
- Easier to determine
- Used for small organisms
3. ECOLOGICAL DENSITY:
- Number of individuals per unit of available habitat
- Accounts for only suitable habitat
- More meaningful than absolute density
FACTORS AFFECTING DENSITY:
- Food and water availability
- Space and shelter
- Temperature and climate
- Predation and disease
- Human activities
POPULATION DISTRIBUTION (DISPERSION):
DEFINITION:
- Pattern of spatial arrangement of individuals in a population
- How individuals are spread across the habitat
TYPES:
1. RANDOM DISTRIBUTION:
- Individuals distributed randomly and independently
- No preference for any area
- Each individual has equal chance of occupying any location
- Occurs when: Resources uniformly distributed, minimal competition, passive dispersal
- Example: Seeds falling from tree and landing randomly
- Least common in nature
2. CLUMPED/AGGREGATED DISTRIBUTION:
- Individuals clustered together in groups
- Higher density in some areas, lower in others
- Most common pattern in nature
- Occurs when: Resources patchily distributed, social behavior, mutual protection
- Example: Herds of zebras, schools of fish, flocks of birds
- Advantages: Collective defense, cooperative foraging, better temperature regulation
3. UNIFORM/REGULAR DISTRIBUTION:
- Individuals evenly spaced throughout habitat
- Relatively constant distance between individuals
- Least common in nature
- Occurs when: Uniform resources, territorial behavior, competition for resources
- Example: Trees in plantation, nesting birds maintaining distance
- Results from territorial behavior and competition
Q3. Describe the different types of age structure in populations and their significance.
Age structure (age pyramid or age distribution) refers to the proportion of individuals of different age groups in a population. It determines population growth patterns and reproductive potential.
Q4. What are limiting factors? Explain density-dependent and density-independent factors.
Limiting factors are environmental factors that restrict population growth and prevent indefinite increase. They determine carrying capacity of environment.
Q5. Explain population growth models: exponential and logistic growth.
Population growth models describe how populations change over time under different environmental conditions.
Q6. What are life tables and survivorship curves? Give their ecological significance.
Life tables are age-specific demographic tables that track survival and mortality of individuals. Survivorship curves graphically represent survival patterns.
Q7. Define ecological succession. Distinguish between primary and secondary succession.
Ecological succession is the gradual process of change in species composition of a community over time, leading toward a stable climax community.
Q8. What is population regulation? Discuss the mechanisms of population control.
Population regulation refers to the mechanisms that maintain populations at relatively stable levels below carrying capacity, preventing indefinite growth.
Organisms and Populations — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following levels of organization includes only members of the same species living in a defined geographical area?
- A. Community
- B. Population
- C. Biome
- D. Ecosystem
Answer: B. Population — A population is a group of individuals of the same species living in a given geographical area at a given time, sharing or competing for similar resources.
MCQ 2. Among the major abiotic factors, which one is considered the most ecologically relevant environmental factor?
- A. Light
- B. Water
- C. Temperature
- D. Soil
Answer: C. Temperature — Temperature is the most ecologically relevant environmental factor as it affects enzyme kinetics, metabolic rate, and other physiological functions of organisms.
MCQ 3. Many desert animals are nocturnal and burrow deep into the sand during the day to avoid the intense heat. This behavior is an example of a:
- A. Physiological adaptation
- B. Behavioral adaptation
- C. Morphological adaptation
- D. Structural adaptation
Answer: B. Behavioral adaptation — Seeking shelter or being active during cooler periods are examples of behavioral responses to cope with unfavorable environmental conditions, making them behavioral adaptations.
MCQ 4. Many zooplankton species in lakes and ponds enter a state of suspended development during unfavorable conditions to survive. This state is known as:
- A. Hibernation
- B. Aestivation
- C. Diapause
- D. Migration
Answer: C. Diapause — Diapause is a stage of suspended development, often observed in zooplankton, that helps them tide over unfavorable conditions like drought or extreme temperatures.
MCQ 5. Why is 'regulation' (maintaining a constant internal environment) considered an energetically expensive process for many organisms?
- A. It reduces their reproductive output.
- B. It requires a lot of metabolic energy to maintain homeostasis.
- C. It makes them more susceptible to predation.
- D. It limits their geographical distribution.
Answer: B. It requires a lot of metabolic energy to maintain homeostasis. — Maintaining a constant body temperature or osmotic concentration requires constant energy expenditure, making regulation an energetically expensive process.
MCQ 6. The number of individuals of a species per unit area or volume is called:
- A. Natality
- B. Mortality
- C. Population density
- D. Population growth
Answer: C. Population density — Population density is a measure of the number of individuals of a particular species per unit area or volume, providing insight into the concentration of a population.
MCQ 7. An age pyramid with a broad base, indicating a high proportion of pre-reproductive individuals, signifies a:
- A. Declining population
- B. Stable population
- C. Expanding population
- D. Zero growth population
Answer: C. Expanding population — A broad-based age pyramid indicates a high number of young individuals who will soon enter the reproductive age, leading to an increasing or expanding population.
MCQ 8. In an exponential growth model, if 'N' is the population density at time 't' and 'r' is the intrinsic rate of natural increase, the population growth rate (dN/dt) is represented by:
- A. dN/dt = rN((K-N)/K)
- B. dN/dt = rN
- C. dN/dt = N(K-N)
- D. dN/dt = K/N
Answer: B. dN/dt = rN — The exponential growth equation is dN/dt = rN, where 'rN' represents the rate of population increase under unlimited resource conditions.
MCQ 9. What does the carrying capacity (K) in a logistic growth curve represent?
- A. The minimum population size required for survival.
- B. The maximum growth rate of the population.
- C. The maximum population size that a given environment can sustain indefinitely.
- D. The intrinsic rate of natural increase.
Answer: C. The maximum population size that a given environment can sustain indefinitely. — Carrying capacity (K) is the maximum population size of a biological species that can be sustained indefinitely by a given environment, considering the available resources.
MCQ 10. Lichens, which are associations between a fungus and an alga (or cyanobacterium), are a classic example of:
- A. Commensalism
- B. Parasitism
- C. Mutualism
- D. Competition
Answer: C. Mutualism — In lichens, both the fungus and the alga benefit from the association; the fungus provides structure and absorption, while the alga performs photosynthesis, making it mutualism.
MCQ 11. What is the primary ecological role of predation?
- A. To increase the population of prey species.
- B. To convert solar energy into chemical energy.
- C. To regulate prey populations and maintain ecosystem stability.
- D. To break down dead organic matter.
Answer: C. To regulate prey populations and maintain ecosystem stability. — Predators play a crucial role in controlling prey populations, preventing overpopulation, and thereby maintaining the balance and stability of the ecosystem.
MCQ 12. According to Gause's Competitive Exclusion Principle, two closely related species competing for the same limited resources cannot co-exist indefinitely. Which of the following mechanisms allows competing species to co-exist?
- A. Parasitism
- B. Resource partitioning
- C. Amensalism
- D. Predation
Answer: B. Resource partitioning — Resource partitioning is a mechanism where competing species evolve to use the same resource in different ways, at different times, or in different places, thereby avoiding direct competition and allowing co-existence.
MCQ 13. Which of the following is an example of an ectoparasite in humans?
- A. Tapeworm
- B. Liver fluke
- C. Head louse
- D. Ascaris
Answer: C. Head louse — Ectoparasites live on the external surface of the host organism. Head lice are a common example of ectoparasites that infest humans, feeding on blood.
MCQ 14. An orchid growing as an epiphyte on a mango tree branch, where the orchid benefits by getting support and exposure to light, while the mango tree is neither harmed nor benefited, exemplifies which type of interaction?
- A. Mutualism
- B. Commensalism
- C. Predation
- D. Parasitism
Answer: B. Commensalism — Commensalism is an interaction where one species benefits, and the other species is neither harmed nor benefited. The orchid benefits from the mango tree's support without affecting it.
MCQ 15. A species that reproduces early in life, produces a large number of small offspring, and has a short lifespan with rapid development is typically categorized as:
- A. K-selected
- B. r-selected
- C. Hibernating
- D. Migratory
Answer: B. r-selected — r-selected species typically produce many small offspring, have short lifespans, reproduce early, and thrive in unstable or unpredictable environments.
MCQ 16. Organisms that can tolerate a wide range of salinities are known as:
- A. Stenohaline
- B. Euryhaline
- C. Osmoconformers
- D. Thermohaline
Answer: B. Euryhaline — Euryhaline organisms are capable of tolerating a wide range of salt concentrations in their environment, enabling them to inhabit varied aquatic habitats.
MCQ 17. The specific physical place or type of environment in which an organism lives is referred to as its:
- A. Niche
- B. Habitat
- C. Biome
- D. Ecosystem
Answer: B. Habitat — A habitat is the natural environment or specific place where an organism normally lives and grows, providing the necessary conditions for its survival.
MCQ 18. The increase in population density due to the number of births in a population during a given period is defined as:
- A. Mortality
- B. Emigration
- C. Natality
- D. Immigration
Answer: C. Natality — Natality refers to the birth rate, which is the number of births in a population during a given period, contributing to an increase in population density.
MCQ 19. Which of the following factors would lead to a decrease in the population density of a given species in a specific area?
- A. Increase in natality and immigration
- B. Decrease in mortality and emigration
- C. Increase in mortality and emigration
- D. Increase in natality and decrease in emigration
Answer: C. Increase in mortality and emigration — Population density decreases when the death rate (mortality) increases, or when individuals move out of the area (emigration).
MCQ 20. For organisms living in very deep oceanic trenches, where sunlight does not penetrate, the primary source of energy is often:
- A. Solar energy from the surface
- B. Thermal energy from hydrothermal vents through chemosynthesis
- C. Energy from decaying organic matter from the surface
- D. Lunar energy
Answer: B. Thermal energy from hydrothermal vents through chemosynthesis — Deep-sea ecosystems are often powered by chemosynthesis, where microorganisms use chemical energy from hydrothermal vents to produce organic matter, serving as the base of the food web.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.