CBSE Class 12 Biology Organisms And Populations Notes

Download CBSE Class 12 Biology Organisms And Populations Notes in PDF format. All Revision notes for Class 12 Biology have been designed as per the latest syllabus and updated chapters given in your textbook for Biology in Standard 12. Our teachers have designed these concept notes for the benefit of Grade 12 students. You should use these chapter wise notes for revision on daily basis. These study notes can also be used for learning each chapter and its important and difficult topics or revision just before your exams to help you get better scores in upcoming examinations, You can also use Printable notes for Class 12 Biology for faster revision of difficult topics and get higher rank. After reading these notes also refer to MCQ questions for Class 12 Biology given our website

Organisms And Populations Class 12 Biology Revision Notes

Class 12 Biology students should refer to the following concepts and notes for Organisms And Populations in standard 12. These exam notes for Grade 12 Biology will be very useful for upcoming class tests and examinations and help you to score good marks

Organisms And Populations Notes Class 12 Biology


Question. If b = 65 and d is = 45, N =100 then find out dN /dt
(a) 2000
(b) 1000
(c) 200
(d) 100
Answer : A

Question. When does the growth rate of a population following the logistic model equal zero? The logistic model is given as dN/dt = rN(l-N/K) NEET 2016
(a) when N nears the carrying capacity of the habitat
(b) when N/K equals zero
(c) when death rate is greater than birth rate
(d) when N/K is exactly one
Answer : D

Question. Which model is considered a more realistic one?
(a) Logistic model
(b) Exponential model
(c) Geometric model
(d) J-shaped model
Answer : A

Question. Asymptote in a logistic growth curve is obtained, when
(a) The value of ‘r’ approaches zero
(b) K = N
(c) K > N
(d) K < N
Answer : B

Question. Populations evolve to maximise their reproductive fitness are also called
(a) Mendel’s fitness
(b) Darwinian fitness
(c) Lamarck’s fitness
(d) Individual fitness
Answer : B

Question. Life history traits of organisms have evolved in relation to the constraints imposed by which components of habitat?
(a) Organic components
(b) Abiotic components
(c) Biotic components
(d) Both (b) and (c)
Answer : D

Question. Choose the incorrect match for life history variations in various organisms.
(a) Breeds only once in their life – Pacific salmon fish, bamboo
(b) Breeds many times during lifetime – Most birds, mammals
(c) Produces large number of small-sized offspring – Birds
(d) Produces large number of large-sized offspring –
Answer : C

Question. Even a plant species, which makes its own food, cannot survive alone, it needs soil microbes to breakdown the …A… matter in soil and return the …B… nutrients for absorption. And then, how will the plant manage pollination without an animal agent? It is obvious that in nature, animals, plants and microbes cannot live in …C… but interact in various ways to form a biological community.
Choose the correct option for A, B and C.
(a) A–inorganic, B–organic, C–isolation
(b) A–organic, B–inorganic, C–isolation
(c) A–organic, B–inorganic, C–community
(d) A–inorganic, B–organic, C–community
Answer : B

Question. Interspecific interactions arise from the interaction of
(a) population of two different species
(b) population of same species
(c) two individuals of same species
(d) two individuals of different area
Answer : A

Question. Interspecific interaction could be
(a) beneficial
(b) detrimental
(c) neutral
(d) All of these
Answer : D

Question. If ‘+’ sign is assigned to beneficial interaction, ‘–’sign to detrimental and ‘0’ sign to neutral interaction,then the population interaction represented by ‘+’ ‘–’refers to
(a) mutualism
(b) amensalism
(c) commensalism
(d) parasitism
Answer : D

Question. The population interaction in which free-living organism that catches, kills and devours individuals of other species called prey is called
(a) parasitism
(b) predation
(c) amensalism
(d) commensalism
Answer : B

Question. Predation is
(a) an unnatural way of transferring of energy to higher trophic level
(b) a natural way of transferring of energy to higher trophic level
(c) harmful to the natural balance
(d) All of the above
Answer : B

Question. Animals eating plants are categorised separately as …A…, they are in a broad ecological context, not very different from …B… .
Choose the correct option A and B.
(a) A–herbivores; B–predator
(b) A–herbivores; B–omnivores
(c) A–omnivores; B–herbivores
(d) A–omnivores; B–predator
Answer : A

Question. Exotic species are also called
I. introduced species
II. alien species
III. non-indigenous species
IV. non-native species
Choose the correct combination.
(a) I, II and III
(b) II, III and IV
(c) I, III and IV
(d) I, II, III and IV
Answer : D

Question. Exotic species sometimes become invasive and starts spreading fast because of
(a) natural predators
(b) abundant natural competitor
(c) invaded land not having its natural predators
(d) mutation in their genome
Answer : C

Question. The prickly pear cactus becomes unusually abundant after its introduction in Australia because it
(a) does not have its predator
(b) formed new mycorrhizal association
(c) lost its thorns
(d) All of the above
Answer : A

Question. Prickly pear cactus (an exotic species) can be brought under control (in Australia) by using
(a) babul eating predators
(b) kikar eating predators
(c) cactus feeding predators
(d) intensive herbicides
Answer : D

Question. Predators also help in …A… species diversity in a community, by …B… the intensity of competition among competing prey species. Here, A and B can be
(a) A–exceeding; B–increasing
(b) A–maintaining; B–reducing
(c) A–reducing; B–maintaining
(d) A–maintaining; B–increasing
Answer : B

Question. Starfish pisaster is the important predator in intertidal communities of
(a) American pacific coast
(b) Indian pacific coast
(c) Middle pacific coast
(d) East Indian lakes
Answer : A

Question. ‘Cryptically-coloured’ (camouflaged) is a technique through which prey can
(a) feed abundantly
(b) lessen the impact of predator
(c) increase their number
(d) increase their reproductive fitness
Answer : B

Question. Monarch butterflies are highly distasteful to predator due to
(a) its ugly look
(b) a special chemical present in his body
(c) Both (a) and (b)
(d) a poison secreted by their special glands
Answer : B

Question. You never see any cattle or goat browsing on Calotropis due to
(a) its appearance
(b) production of foul odour
(c) production of cardiac glycosides
(d) distastefulness of its leaves
Answer : C

Question. Which of the following is not an example of a defence used by plants against herbivores?
(a) Production of caffeine, tannins and quinine
(b) More production of non-woody tissues
(c) Production of hairs, thorns, spines
(d) Production of hormone-like chemicals that interfere with insect metamorphosis
Answer : B

Question. Which of the following is not an example of prey-predator relationship?
(a) Tiger eating deer
(b) Plant Nepenthes trapping an insect
(c) Bacteria decomposing organic matter
(d) Crocodile killing a man
Answer : C

Question. In which of the following interactions both partners are adversely affected?
(a) Competition
(b) Predation
(c) Parasitism
(d) Mutualism
Answer : A

Question. On the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal areas and excludes the smaller barnacle Chathamalus from that zone.
Which kind of interaction is being depicted by this example?
(a) Predator
(b) Parasitism
(c) Commensalism
(d) Competition
Answer : D

Question. Level of competition between species depends on
(a) availability of resources
(b) population density
(c) group interaction of organism
(d) All of the above
Answer : D

Question. When Darwin spoke of the struggle for the existence and survival of the fittest in the nature, he was convinced that
(a) intraspecific competition is a potent force in organic evolution
(b) interspecific competition is a potent force in organic evolution
(c) intensive reproduction is the potent force in organic evolution
(d) intensive predation is the potent force in organic evolution
Answer : B

Question. Competition is best defined as a process in which the fitness of one species (measured in terms of its ‘r’ the intrinsic rate of increase) is significantly
(a) lower in the presence of another superior species
(b) higher in the presence of another superior species
(c) equal in the presence of another superior species
(d) equal in the presence of their own species
Answer : A

Question. A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species is found to expand its distributional range dramatically when the competing species is experimentally removed. This is called as
(a) competitive exclusion
(b) competitive release
(c) predation
(d) mutualism
Answer : A

Question. The principle of competitive exclusion was stated by
(a) C Darwin
(b) GF Gause 
(c) MacArthur
(d) Verhulst and Pearl
Answer : B

Question. Gause’s principle of competitive exclusion states that
(a) competition for the same resources excludes species having different food preferences
(b) no two species can occupy the same niche indefinitely for the same limiting resources
(c) larger organisms exclude smaller ones through competition
(d) more abundant species will exclude the less abundant species through competition
Answer : B

Question. Species facing competition might evolve mechanism that promotes coexistence rather than exclusion. One such mechanism is
(a) competitive release
(b) resource partitioning
(c) coevolution
(d) None of the above
Answer : B

Question. Read the following reasons for the adaptation in parasites.
I. loss of unnecessary organs.
II. presence of adhesive organs.
III. origin of suckers to cling to host.
IV. loss of digestive system.
V. high reproductive capacity.
Choose the correct option.
(a) I, III and IV
(b) II, IV and V
(c) I, IV and V
(d) I, II, III, IV and V
Answer : D

Question. Parasite that feed on the external surface of the host organism is called
(a) endoparasite
(b) ectoparasite
(c) brood parasite
(d) None of these
Answer : B

Question. ....A... parasite have simple life cycle and ....B....parasite have complex life cycle.
(a) A–Ecto, B–endo
(b) A–Endo, B–brood
(c) A–Brood, B–endo
(d) A–Brood, B–ecto
Answer : A

Question. An interaction in which one organism is benefitted and other is unaffected
(a) predation
(b) commensalism
(c) mutualism
(d) parasitism
Answer : B


Ecology: It is a branch of biology which studies the interactions i) among organisms and ii) between the organism and its physical ( abiotic) environment.

Organism: Individual of a species.

Population: Population is a group of individuals of the same species which can interbreed among themselves and live in a localized area.

Biome: The largest ecological regions distinguishable by characteristic plants and animals. Eg. tundra, conifer, deciduous forest, grassland, tropical, and desert.

Alpine tundra: Alpine tundra is located on mountains throughout the world at high altitude where trees cannot grow. The growing season is approximately 180 days. The night time temperature is usually below freezing. Unlike the arctic tundra, the soil in the alpine is well drained.

Group of animals that live in Alpine tundra:

Mammals: mountain goats, sheep

Insects: beetles, grasshoppers, butterflies

Coniferous forests: Coniferous forests are made up mainly of cone-bearing or coniferous trees, such as spruces, hemlocks, pines and firs. The leaves of these trees are either small and needle-like or scale-like and most stay green all year around (evergreen). All are softwoods able to survive cold temperatures and acidic soil.

Temperate Forests: The term ‘temperate forest’ is very broad. It covers the forests found between the tropical and subtropical regions and the barren, treeless lands of the far north and extreme south.

Tropical forest: "Tropical forests" encompass the idyllic rainforest, the remote cloud forest, and the lesser-known but equally endangered dry forest, pine savanna and much, much more. They are not one ecosystem, but millions of unique ecosystems.

Deciduous forest: The average annual temperature in a deciduous forest is 50° F. The average rainfall is 30 to 60 inches a year. The deciduous forest has four distinct seasons, spring, summer, autumn, and winter. In the autumn the leaves change color. During the winter months the trees lose their leaves. 

Temperature, water, light, soil.


• Average temperature varies seasonally

  Organisms may be Eurythermal or Stenothermal

  Eurythermal- wide range of temperature tolerence

 Stenothermal-Narrow range of temperature tolerance 


 Influences life of organisms. No life without water.

 Productivity and distribution of plants are water dependent.

 Organisms may be Euryhaline or Stenohaline.

  Euryhaline:Wide range of salinity tolerance

  Stenohaline:Narrow range of salinity tolerance. 


  Photosynthesis and release of oxygen light dependent.

  Sciophytes need to use diurnal and seasonal light intensity of forage, migration and reproduction. 


  Nature and proportion of soil in a place depends on climate, weathering process and types of soil.

  Soil composition, grain size and aggregation determine percolation and water holding capacity of soil.

 Physical and chemical properties determine type of plants and Animals that survive in a habitat. 

Response of organisms to environmental condition 



 Organisms maintain homeostasis achieved by physiological and behavioral means
 Thermo regulation and osmoregulation.

 Cannot maintain constant internal Environment
 Body temperature and osmotic concentration of body changes with ambient temperature and concentration of medium.-Thermo confirmer and osmo-confirmer

 Organism moves away temporarily to another habitat in stressful condition. e.g.- Migratory birds like Siberian crane

 Organisms suspend their metabolic activities during stressful condition
 Resume their function at the return of favorable conditions.

E.g. Hibernation (winter sleep) of Frog, Reptiles, Polar Bear etc
 Aestivation (summer sleep) in Snail and Fish.
 Seed dormancy.

 Morphological, physiological and behavioral changes that enable organisms to adjust to the ever changing environment .

E.g. Kangaroo rat survives in desert conditions through internal oxidation of fat, removing concentrated urine of limited quantity.
 Allen‗s rule-cold climate mammals have shorter ears and limbs to minimize heat loss.
 Polar mammals like seals have blubber to prevent heat loss.
 Burrowing habit to escape from heat
 Higher count of RBC, Hb (haemoglobin) at high altitudes.

Population attributes
 Birth Rate – Number of individuals born per thousand per year.
 Death Rate – Number of individuals die per thousand per year.
 Sex Ratio – Ratio of male-female in the population.
 Population density. - the number of individual organisms per unit area (appropriate measure – total number-sometimes difficult to determine or meaningless because 4
factors N+I-M+E are concerned w.r.t habitat concerned )

Age pyramids
 If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid.
 Three ecological ages:
 Pre-reproductive, Reproductive and Post-Reproductive
 High proportion pre-reproductive individuals occur in Expanding population
 Pre-reproductive individuals are uniform in Stable population.
 Pre-reproductive individuals are less in Declining population.

CBSE Class 12 Biology - Organisms and Populations


Population growth

Factors that affect the size of population
Predation pressure
Density of population at any time at a given
place depends on Natality, Mortality, Emigration

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