Question

DDT was sprayed on some insects as an experiment. Among them, some died and some survived. A second generation was created by mating the survived ones. Again, DDT was sprayed. This process was repeated over five generations. The result is given in the table. Analyse it and answer the following questions.

Generations	Percentage of survived insects
1	10
2	20
3	30
4	40
5	50

(a) Interpret the result given in the table.
(b) What scientific explanation can you give for this result?
(c) What may be the result if the experiment is continued?

Answer 1

Hint: DDT stands for dichlorodiphenyltrichloroethane. It is an insecticide which is hazardous for the environment as it cannot be decomposed easily. It does not occur naturally but has to be synthesised artificially.

Complete answer:
DDT is an organic pollutant which is readily adsorbed by the soil and affects organisms in the long term. DDT is used as an agricultural insecticide. It was also used to control malaria and dengue insects. DDT was stopped to be used in various programs as the insects were seen to acquire tolerance against DDT. It was also seen that the use of DDT is less effective in the tropical areas.

1)According to the result shown in the table, the percentage of surviving insects increases through subsequent generations. It means that the resistance to DDT pesticide increases. This shows that the mutation has occurred in the insect.

2)The above result is in accordance with Darwin's theory of Natural selection. It states that the organisms which have evolved traits are selected over the ones with less superior traits. The new generation are better adapted to the environment. So these changes are transmitted to the next generations.

3)The table shows that the resistance increases in subsequent generations. It is quite possible that the insects become resistant in future generations against DDT.
Note: The lipophilic nature of the DDT leads to bioaccumulation in the predatory birds. It is also toxic to living organisms like marine animals, crayfish, sea shrimp and several fishes. They enter the food chain and lead to magnification. It gets stored in the body fat and is resistant to metabolism.