Read the given passage and answer the following questions:
During post-industrialisation period, the tree trunks became dark due to industrial smoke and soots. Under this condition the white-winged moth did not survive due to predators, dark-winged or melanised moth survived. Before industrialisation set in, thick growth of almost white-coloured lichen covered the trees - in that background the white winged moth survived but the dark-coloured moth were picked out by predators. Do you know that lichens can be used as industrial pollution indicators? They will not grow in areas that are polluted. Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived. This understanding is supported by the fact that in areas where industrialisation did not occur e.g., in rural areas, the count of melanic moths was low. This showed that in a mixed population, those that can better-adapt, survive and increase in population size. Remember that no variant is completely wiped out. Similarly, excess use of herbicides, pesticides, etc., has only resulted in selection of resistant varieties in a much lesser time scale. This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell. Hence, resistant organisms/cells are appearing in a time scale of months or years and not centuries. These are examples of evolution by anthropogenic action. This also tells us that evolution is not a directed process in the sense of determinism. It is a stochastic process based on chance events in nature and chance mutation in the organisms.
The rise in the proportion of dark-winged moths after industrialisation shows natural selection leading to:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Directional change
Natural selection can act on a trait in three ways. In stabilising selection, more individuals come to have the mean (average) value of the trait. In directional selection, more individuals come to have a value away from the mean, so the peak of the curve shifts to one side. In disruptive selection, the individuals at both ends of the range are favoured and two peaks form. After industrialisation the dark-winged moths were favoured on the sooty tree trunks, and their share rose year after year. The population moved in one direction, from mostly white to mostly dark.
Option (1) Stabilisation — Incorrect. Stabilisation favours the average form and makes the peak higher and narrower. The moth population did not settle around its old mean; it shifted towards the dark form.
Option (2) Directional change — Correct. The dark form, which was rare before, became common. More individuals came to have a value other than the mean, which is directional change.
Option (3) Disruption — Incorrect. Disruption favours both extreme forms at once and gives two peaks. Only the dark form was favoured on the dark trunks, so there was one shift, not a split.
Option (4) Gene flow — Incorrect. Gene flow is the movement of genes from one population to another through migration. It is one factor that disturbs Hardy-Weinberg equilibrium, but it is not a result of natural selection.
NCERT Extract:
Hardy-Weinberg Principle – “… Coupled to enhance reproductive success, natural selection makes it look like different population. Natural selection can lead to stabilisation (in which more individuals acquire mean character value), directional change (more individuals acquire value other than the mean character value) or disruption (more individuals acquire peripheral character value at both ends of the distribution curve) (Figure 6.8).”
Figure 6.8 Diagrammatic representation of the operation of natural selection on different traits : (a) Stabilising (b) Directional and (c) Disruptive