In a laboratory, if 50 Drosophila out of a population of 250 die in a week then the death rate of Drosophila is _________ individual per Drosophila per week.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2.0
Note: The given answer is as per the official NTA Answer Key. However, the correct answer should be Option (1) 0.2, as explained below.
The death rate of a population is a per capita rate: the number of individuals that die in a given period divided by the number of individuals present in the population. It tells how many deaths occur per member of the population, not the total number of deaths.
Here, 50 Drosophila die in one week out of a population of 250:
Death rate = number that died ÷ size of the population = 50 ÷ 250 = 0.2 individuals per Drosophila per week.
So one fly in every five died during that week. A per capita death rate can never be more than 1, because no more flies can die than are present.
Why the answer given by NTA is incorrect
Option (1) 0.2 — Correct as per NCERT. Deaths divided by the population: 50/250 = 0.2 individuals per Drosophila per week, the same per capita method NCERT uses for fruitflies (4/40 = 0.1).
Option (2) 2.0 — The answer given by NTA, but incorrect. It is ten times the true value; a per capita death rate can never exceed 1 in this case, since no more than all 250 flies can die.
NCERT Extract:
Population Attributes – “A population has certain attributes whereas, an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths. The rates, hence, expressed are change in numbers (increase or decrease) with respect to members of the population. Here is an example. If in a pond there were 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. If 4 individuals in a laboratory population of 40 fruitflies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruitfly per week.”