NEET Biology — Organisms and Populations Complete Chapter Guide

Chapter Overview & Weightage

Organisms and Populations covers how organisms adapt to their environment, population attributes (size, density, growth), growth models, life history variation, and population interactions (competition, predation, parasitism, mutualism, commensalism).

This chapter carries 3-4% weightage in NEET with 2-3 questions. Population growth models and interspecific interactions are the highest-yield topics.

Key Concepts You Must Know

Tier 1 (Core)

Population attributes: size ( $N$ ), density, birth rate ( $b$ ), death rate ( $d$ ), age structure, sex ratio
Population growth: exponential ( $dN/dt = rN$ ) vs logistic ( $dN/dt = rN(K-N)/K$ )
Carrying capacity ( $K$ ): maximum population size the environment can sustain
Population interactions: mutualism (+/+), competition (-/-), predation (+/-), parasitism (+/-), commensalism (+/0), amensalism (-/0)

Tier 2 (Frequently tested)

Adaptations: morphological, physiological, behavioural
Thermoregulation: endotherms (regulate body temp internally) vs ectotherms (body temp varies with environment)
Allen’s rule, Bergmann’s rule (cold-climate adaptations)
Age pyramids: expanding (triangular), stable (bell-shaped), declining (urn-shaped)

Tier 3 (Occasionally tested)

r-selected vs K-selected species
Competitive exclusion principle (Gause’s principle)
Brood parasitism (cuckoo lays eggs in crow’s nest)

Important Formulas

Exponential growth (unlimited resources):

\frac{dN}{dt} = rN

Integrated form: $N_t = N_0 e^{rt}$

Graph: J-shaped curve

Logistic growth (limited resources):

\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)

Graph: S-shaped (sigmoid) curve, levels off at carrying capacity $K$

Where:

$N$ = population size
$r$ = intrinsic rate of natural increase
$K$ = carrying capacity
$t$ = time

Interaction	Species A	Species B	Example
Mutualism	+	+	Lichens (fungus + alga), mycorrhiza
Competition	-	-	Flamingoes and resident fish (Galapagos)
Predation	+ (predator)	- (prey)	Tiger and deer
Parasitism	+ (parasite)	- (host)	Cuscuta on host plant, liver fluke in sheep
Commensalism	+	0	Orchid on mango tree (epiphyte), barnacles on whale
Amensalism	0	-	Penicillium inhibits Staphylococcus

NEET loves the logistic growth equation. Remember: when $N$ is small compared to $K$ , $(K-N)/K$ is close to 1, so growth is nearly exponential. When $N$ approaches $K$ , $(K-N)/K$ approaches 0, and growth stops. Maximum growth rate occurs at $N = K/2$ .

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: The

sigmoid growth curve is shown by population following:

(A) Exponential growth (B) Logistic growth (C) Arithmetic growth (D) Geometric growth

Solution:

The sigmoid (S-shaped) curve is characteristic of logistic growth, where the population initially grows exponentially but then slows down as it approaches the carrying capacity ( $K$ ) due to resource limitation.

Exponential growth gives a J-shaped curve (no upper limit).

Answer: (B) Logistic growth

PYQ 2 — NEET 2023

Problem: An orchid growing on a mango tree is an example of:

(A) Parasitism (B) Mutualism (C) Commensalism (D) Competition

Solution:

The orchid (epiphyte) uses the mango tree only for physical support — it gets better access to light. The mango tree is neither benefited nor harmed. This is commensalism (+/0 interaction).

If the orchid were taking nutrients from the tree, it would be parasitism. But epiphytes make their own food through photosynthesis.

Answer: (C) Commensalism

PYQ 3 — NEET 2022

Problem: According to Gause’s competitive exclusion principle:

(A) Two species can coexist in the same niche (B) Two closely related species competing for the same resources cannot coexist indefinitely (C) Competition always leads to extinction (D) Predation prevents competition

Solution:

Gause’s principle states that two species competing for the same resources cannot coexist indefinitely in the same habitat — one will be competitively superior and eliminate the other. Coexistence requires resource partitioning (occupying different niches).

Answer: (B) Two closely related species competing for the same resources cannot coexist indefinitely

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	45%	Interaction type identification, growth curve shape
Medium	40%	Growth equation application, adaptation examples
Hard	15%	Competitive exclusion, r vs K selection

Expert Strategy

Day 1: Population growth models — understand both equations, draw both curves, know when each applies. Practice the logistic growth equation with numerical values.

Day 2: Population interactions — make the +/- table and memorise one clear example for each interaction type. Focus on distinguishing parasitism from predation and commensalism from mutualism.

Day 3: Adaptations and age pyramids. Know Allen’s rule (shorter extremities in cold) and Bergmann’s rule (larger body in cold). Understand the three age pyramid shapes and what each means for population growth.

The carrying capacity concept connects to many real-world NEET questions. If asked “what limits population growth?” — the answer is environmental resistance (limited food, space, disease, predation). These factors collectively define $K$ .

Common Traps

Trap 1 — Exponential growth gives J-curve, logistic gives S-curve. Don’t swap them. J-shaped = no limit = exponential. S-shaped (sigmoid) = limited by K = logistic.

Trap 2 — Commensalism means one benefits, the other is UNAFFECTED (not harmed). If both benefit, it’s mutualism. If one is harmed, it’s parasitism or predation. The “0” in commensalism means truly neutral — no effect on the host.

Trap 3 — Parasitism is +/-, just like predation, but the parasite usually doesn’t kill the host. Parasites depend on the living host for nutrition. Predators kill and consume prey. NEET may ask you to distinguish between the two despite both being +/-.