CBSE Class 12-science Answered

In directional selection, one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. In situation I, since peccaries feed on cacti with low number of spines, there was a selection pressure against these cacti. As a result, the cacti with high number of spines were favoured by nature and their population increased. This means that one extreme trait got selected by nature. This clearly shows directional selection in action.

When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection. In situation II, since the peccaries feed on cacti with low number of spines, there was a selection pressure against these cacti. At the same time, since the insect species were parasitic on cacti with high number of spines, there was a selection pressure against these cacti as well. Combined, these two selection pressures select to maintain cacti with intermediate spines. The number of cacti with intermediate spines will increase while the numbers of cacti with low and high number of spines will decrease. This clearly shows stabilising selection in action.

So, the correct answer is option 3 and not option 2.