Biology · Evolution · NEET
Look at what the trait mean does. Stabilising: more individuals get the MEAN (average) value, variation decreases, one central peak stays. Directional: more individuals get a value OTHER than the mean at one extreme, so the mean shifts to one side. Disruptive: more individuals get BOTH extreme values, the mean stays but the middle empties out, giving two peaks. NCERT (Ch 6 Evolution) states these three clearly: stabilisation = mean value, directional change = value other than the mean, disruption = both extremes.
Stabilising selection favours the mean (average) character value. It removes both the very low and very high extremes, so variation in the population goes down and the population stays close to its current average. The classic NEET example is human birth weight: babies of average weight survive best, while very small and very large babies have lower survival.
Stabilising selection. Newborns with average weight (about 3 to 3.5 kg) survive at the highest rate, while babies at both extremes (too light or too heavy) have higher mortality. Because the mean value is favoured and both extremes are removed, this is stabilising selection, not directional or disruptive. This exact idea was asked in NEET 2019.
Directional. When breeders keep choosing only the highest milk-yielding cows generation after generation, one extreme (high yield) is favoured and the population mean shifts toward higher yield. That is directional selection. It is not disruptive, because disruptive would keep BOTH extremes (very high AND very low yield). This was asked directly in NEET 2017.
Disruptive selection favours both extreme values of a trait and selects against the average (middle) individuals. Over generations the middle of the population shrinks and the two extremes grow, so the frequency curve develops two separate humps (bimodal). This can eventually split one population into two distinct groups and is linked to speciation.
Directional selection SHIFTS the mean toward one extreme; it does not simply increase variation. One end of the trait range is favoured (for example larger size or higher yield), so over generations the whole population average moves in that one direction. Remember: 'directional' = one direction = mean slides sideways.
In a species, the weight of newborn ranges from 2 to 5 kg. 97% of the newborn with an average weight between 3 to 3.3 kg survive whereas 99% of the infants born with weight from 2 to 2.5 kg or 4.5 to 5 kg die. Which type of selection process is taking place?
Artificial selection to obtain cows yielding higher milk output represents:
Natural selection where more individuals acquire a specific character value other than the mean character value, leads to:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Stabilising selection (favours the mean value, removes both extremes), directional selection (favours one extreme, shifts the mean to one side), and disruptive selection (favours both extremes, removes the middle and can create two groups). These three are shown in NCERT Class 12 Chapter 6, Evolution.
Stabilising selection reduces variation because it keeps the population near its average. Directional selection shifts the average but keeps variation moderate. Disruptive selection increases variation the most, since it splits the population toward both extremes and can lead to two distinct forms.
Yes. Because disruptive selection pushes a population toward two different extreme forms and empties the middle, it can eventually divide one population into two, which is a step toward speciation. This links to the next topic, speciation.
Use the peak shapes: Stabilising = one tall central peak (narrow), Directional = the single peak slides to one side, Disruptive = two peaks (bimodal, like a camel's two humps). Match the NEET question's description of the mean to the correct shape.
