Organisms and Populations NEET Notes 2027: Key Points and PDF

Organisms and Populations is Chapter 11 of NCERT Class 12 Biology and part of Unit 10, Ecology and Environment, of the NEET UG syllabus. These notes cover population attributes, age pyramids, population growth, exponential and logistic models, life history variation and population interactions such as predation, competition, parasitism, commensalism and mutualism, with a free notes PDF.

Organisms and Populations NEET Notes

Section-wise key points from NCERT Chapter 11, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Organisms and Populations at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

What does this chapter cover in the rationalised NCERT book?

The rationalised Chapter 11 has a single section, 11.1 Populations, with four parts: population attributes, population growth, life history variation and population interactions.

• Ecology studies interactions among organisms and between organisms and their physical (abiotic) environment, at four levels: organisms, populations, communities and biomes.
• Ramdeo Misra (1908-1998) is revered as the Father of Ecology in India; he set up ecology teaching and research at Banaras Hindu University.
• ‘How-type’ questions seek the mechanism of a process; ‘why-type’ questions seek its significance.
• Natural selection operates at the population level, so population ecology links ecology to population genetics and evolution.
• Older notes may also cover habitat, biomes, abiotic factors and adaptations; these are not in the current NCERT chapter.

What are the attributes of a population?

A population is a group of individuals of a species living in a defined area, sharing or competing for similar resources and potentially interbreeding. Examples: cormorants in a wetland, teakwood trees in a forest tract, bacteria in a culture plate, lotus plants in a pond.

• An individual has births and deaths; only a population has birth rates, death rates, sex ratio and age distribution.
• Density need not be a number: per cent cover or biomass is more meaningful when one huge banyan tree shares an area with 200 carrot grass (Parthenium hysterophorus) plants.
• Relative density is often enough, for example the number of fish caught per trap.
• Populations are often estimated indirectly; tiger census in national parks and tiger reserves uses pug marks and fecal pellets.

How does a population grow?

Population density changes through four processes: natality and immigration increase it, and mortality and emigration decrease it.

• N_t+1 = N_t + [(B + I) – (D + E)].
• Under normal conditions births and deaths matter most; immigration can matter more when a new habitat is being colonised.

Exponential and Logistic Growth

• r = b – d, the intrinsic rate of natural increase; it is used to assess the impact of biotic or abiotic factors on growth.
• NCERT r values: Norway rat 0.015, flour beetle 0.12, human population of India in 1981 0.0205.
• K is the carrying capacity, the maximum number a habitat can support.
• Darwin showed that even a slow-breeding elephant could reach enormous numbers without checks; NCERT also uses the chessboard wheat story and a Paramecium doubling every day.

What is life history variation?

Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high r value), in their habitat.

• Life history traits evolve in relation to the constraints of the abiotic and biotic components of the habitat.

What are the types of population interactions?

Interspecific interactions are beneficial (+), detrimental (–) or neutral (0) to each species. No habitat holds just one species.

• Predation, parasitism and commensalism share one feature: the interacting species live closely together.

Predation

• Predation transfers energy fixed by plants to higher trophic levels; herbivores are, in a broad sense, predators of plants.
• Predators keep prey populations under control. The prickly pear cactus introduced into Australia in the early 1920s spread over millions of hectares until a cactus-feeding moth was introduced. Biological pest control is based on this ability.
• Predators maintain species diversity by reducing competition among prey: removing the starfish Pisaster from an intertidal area of the American Pacific Coast made more than 10 invertebrate species extinct within a year.
• Predators are ‘prudent’; overexploiting prey can make both prey and predator extinct.
• Prey defences: camouflage (some insects and frogs), being poisonous, and the Monarch butterfly, distasteful to birds because of a chemical it gets as a caterpillar from a poisonous weed.
• Nearly 25 per cent of insects are phytophagous. Plant defences: thorns (Acacia, Cactus) and chemicals such as the cardiac glycosides of Calotropis; nicotine, caffeine, quinine, strychnine and opium are also anti-herbivore chemicals.

Competition

• Unrelated species can compete, and resources need not be limiting: in interference competition one species lowers the feeding efficiency of another even when food and space are abundant.
• Competition is best defined as a process in which the fitness (r) of one species is significantly lower in the presence of another.
• Gause’s Competitive Exclusion Principle: two closely related species competing for the same resources cannot co-exist indefinitely; the inferior one is eliminated. This may hold if resources are limiting, but not otherwise.
• Evidence: the Abingdon tortoise of the Galapagos became extinct within a decade of goats being introduced.
• Competitive release: Connell showed that on the rocky coasts of Scotland the larger barnacle Balanus excludes the smaller Chthamalus (spelt Chathamalus in NCERT) from the intertidal zone.
• Herbivores and plants appear more adversely affected by competition than carnivores.
• Resource partitioning allows co-existence: MacArthur showed five closely related warbler species on the same tree avoid competition through different foraging behaviour.

Parasitism

• Many parasites are host-specific, and host and parasite tend to co-evolve.
• Adaptations: loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system and high reproductive capacity.
• The human liver fluke needs two intermediate hosts (a snail and a fish); the malarial parasite needs a vector (mosquito).
• Ectoparasites feed on the host’s surface: lice on humans, ticks on dogs, copepods on marine fish, and Cuscuta, which has lost its chlorophyll and leaves. The female mosquito is not considered a parasite.
• Endoparasites live inside the host (liver, kidney, lungs, red blood cells) and have simplified bodies with high reproductive potential.
• Brood parasitism: the cuckoo (koel) lays eggs in the crow’s nest; its eggs have evolved to resemble the host’s eggs in size and colour.

Commensalism and Mutualism

• Commensalism: orchid growing as an epiphyte on a mango branch; barnacles on the back of a whale; cattle egret foraging near grazing cattle that flush out insects; clown fish living among the stinging tentacles of a sea anemone.
• Mutualism: lichens (fungus with photosynthesising algae or cyanobacteria) and mycorrhizae (fungi with roots of higher plants; fungi help absorb nutrients, the plant gives carbohydrates).
• Fig and wasp: a one-to-one relationship; the female wasp pollinates the fig inflorescence while laying eggs and its larvae feed on some developing seeds.
• The Mediterranean orchid Ophrys uses ‘sexual deceit’: a petal resembles the female bee, and the male bee pollinates while ‘pseudocopulating’ with the flower.
• Plant-animal mutualisms often involve co-evolution of flower and pollinator.

Most-asked Facts for NEET: Organisms and Populations

Revise these one-line facts from the NCERT chapter just before the exam.

• Ramdeo Misra is revered as the Father of Ecology in India.
• Ecology has four levels of organisation: organisms, populations, communities and biomes.
• Birth and death rates are per capita: 8/20 = 0.4 for lotus, 4/40 = 0.1 for fruitflies.
• Age pyramids show whether a population is growing, stable or declining.
• Tiger census uses pug marks and fecal pellets.
• N_t+1 = N_t + [(B + I) – (D + E)].
• Exponential growth: dN/dt = rN, J-shaped curve; logistic growth: dN/dt = rN[(K – N)/K], sigmoid curve.
• r = b – d; Norway rat 0.015, flour beetle 0.12, India (1981) 0.0205.
• K is the carrying capacity; logistic growth is Verhulst-Pearl growth.
• Pacific salmon and bamboo breed only once in their lifetime.
• Mutualism +/+, competition –/–, predation and parasitism +/–, commensalism +/0, amensalism –/0.
• Prickly pear cactus in Australia was controlled by a cactus-feeding moth.
• Removing starfish Pisaster made more than 10 invertebrate species extinct within a year.
• Calotropis produces cardiac glycosides; Monarch butterfly is distasteful to birds.
• Gause: Competitive Exclusion Principle; Connell: Balanus and Chthamalus; MacArthur: warblers and resource partitioning.
• Abingdon tortoise became extinct after goats were introduced to the Galapagos.
• Cuckoo (koel) shows brood parasitism in the crow’s nest.
• Cattle egret and cattle, clown fish and sea anemone are commensalism; fig and wasp, Ophrys and bee are mutualism.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Calling the female mosquito a parasite. NCERT says it is not considered a parasite even though it needs blood for reproduction.
• Swapping the curves: exponential growth is J-shaped; logistic growth is S-shaped (sigmoid).
• Using total counts as the only measure of density. Per cent cover, biomass or relative density can be more meaningful.
• Saying competition happens only between closely related species for limiting resources. Unrelated species compete too, and interference competition occurs even with abundant resources.
• Calling the orchid on a mango tree a parasite. It is an epiphyte and the interaction is commensalism.
• Labelling lichens or mycorrhizae as commensalism. Both are mutualism.
• Writing r = b + d. The intrinsic rate of natural increase is r = b – d.
• Forgetting that herbivores are predators of plants in the broad ecological sense.
• Mixing up Balanus and Chthamalus: the larger Balanus is the superior competitor that excludes the smaller Chthamalus.
• Revising habitat, biomes and abiotic-factor adaptations from old notes as part of this chapter. The rationalised NCERT chapter covers populations only.

Organisms and Populations Notes PDF and Related Pages

Download the AglaSem notes for Organisms and Populations (key notes and important questions with answers) free: View PDF

• All chapters: NEET Biology notes
• Full topic list: NEET syllabus
• Practice: NEET question papers
• Source text: NCERT Chapter 11 (NCERT, Class 12 Biology)

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