Organisms and Populations — Questions & Answers

Sample questions from Organisms and Populations

1. Q1. Prey defences against predation include morphological, behavioural and chemical strategies. Which set is correctly assigned?

   • A.Camouflage is chemical, while thorns are behavioural
   • B.Cardiac glycosides in Calotropis are morphological, while thorns are chemical
   • C.Being distasteful is morphological, while camouflage is chemical
   • D.Camouflage is morphological, while storing a milkweed toxin is chemical✓
   Solution

   Cryptic coloration is a structural or morphological defence. The Monarch's unpalatability comes from a chemical sequestered from milkweed during the caterpillar stage. Thorns in Acacia and cactus are morphological, and cardiac glycosides in Calotropis are chemical.

2. Q2. Two populations of the same species obey dN/dt = rN(K-N)/K with r = 0.3 and K = 1000. Population P has N = 150 and population Q has N = 850. Comparing their instantaneous growth rates:

   • A.Q grows faster than P because it has more breeding individuals
   • B.Q has a negative dN/dt because N exceeds K/2
   • C.P grows faster than Q because it is further from K
   • D.Both have the same dN/dt of about 38.25 individuals per year✓
   Solution

   The logistic expression is symmetrical about K/2, so N and K - N give identical rates: 0.3 x 150 x 850/1000 = 38.25 and 0.3 x 850 x 150/1000 = 38.25 per year. Q is not declining, since N is still below K.

3. Q3. A person flown to 3500 metres altitude feels breathless initially but adjusts within a week. The correct sequence of physiological changes underlying this acclimatisation is:

   • A.Low atmospheric temperature triggers shivering, which raises oxygen delivery
   • B.Reduced haemoglobin affinity for oxygen allows more oxygen to be loaded in the lungs
   • C.Low partial pressure of oxygen leads to increased red blood cell production and a rise in breathing rate, with increased binding affinity of haemoglobin for oxygen✓
   • D.High partial pressure of oxygen leads to fewer red blood cells and slower breathing
   Solution

   At high altitude the low partial pressure of oxygen causes altitude sickness. The body compensates by producing more erythrocytes, increasing the binding affinity of haemoglobin for oxygen and breathing faster, all of which improve oxygen delivery to tissues.

4. Q4. A population of 2000 individuals records 250 births and 150 deaths in one year, with no migration. Its per capita growth rate r and, if exponential growth continued, its size after 30 years would be about:

   • A.r = 100 per year and about 5000 individuals
   • B.r = 0.20 per year and about 8963 individuals
   • C.r = 0.05 per year and about 8963 individuals✓
   • D.r = 0.05 per year and about 5000 individuals
   Solution

   Per capita birth rate = 250/2000 = 0.125; per capita death rate = 150/2000 = 0.075; r = 0.05 per year. Nt = 2000 x e^(0.05 x 30) = 2000 x e^1.5 = 2000 x 4.4817 = about 8963.

5. Q5. A population showing a "J-shaped" growth curve over time, when plotted as population size against time, is characteristic of:

   • A.Exponential growth under unlimited resource conditions✓
   • B.Logistic growth that has stabilized at carrying capacity
   • C.A population in continuous decline
   • D.A population with zero growth rate throughout
   Solution

   The J-shaped curve is the signature graphical pattern of exponential growth, showing continuously accelerating population increase with no leveling off, in contrast to the S-shaped (sigmoid) curve of logistic growth.

Other Class 12 Biology chapters