Read the given passage and answer the questions that follow :-
In a given population one can find out the frequency of occurrence of alleles of a gene or a locus. This frequency is supposed to remain fixed and even remain the same through generations. Hardy-Weinberg principle stated it using algebraic equations. This principle says that allele frequencies in a population are stable and is constant from generation to generation. The gene pool remains a constant. This is called genetic equilibrium.
The graph given below shows how the distribution of a trait in a population has changed. Which of the following does it show?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Disruptive selection
Natural selection can change the spread of a trait in a population in three ways. In the graph the single peak has given way to two peaks at the two ends of the curve, with fewer individuals in the middle. This means more individuals now carry peripheral character values at both ends of the distribution.
Option (1) Disruptive selection — Correct. Disruptive selection favours both extremes, so two peaks form, as in Figure 6.8(c).
Option (2) Directional selection — Incorrect. In directional selection the single peak shifts in one direction, because more individuals acquire a value other than the mean.
Option (3) Stabilising selection — Incorrect. In stabilising selection more individuals acquire the mean value, so the single peak gets higher and narrower.
Option (4) Genetic drift — Incorrect. Genetic drift is a chance change in allele frequencies. It is not one of the three patterns of selection drawn in this figure.
Note: Figure 6.8 packs three kinds of selection into three small graphs, and a single panel of it was enough for this question. Learn the shape of each curve along with its name.
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