The graph shows highest survival at intermediate birth weight and lower survival at both low and high weights, so the mode of selection on human birth weight is stabilizing selection.

Introduction

In evolutionary biology, the relationship between human birth weight and infant survival is a classic textbook example of stabilizing natural selection. IIT JAM 2023 Biotechnology Question 29 tests this concept using a graph that plots degree of survival against birth weight for 6908 newborns in a hospital. Understanding the shape of this curve and the options given helps in mastering modes of selection and improving exam performance.

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Understanding the given graph

• The curve is a single-peaked, inverted parabola (bell-shaped) with maximum degree of survival at an intermediate birth weight (around 2.5–3.5 kg). Low and high birth weights on either side of the optimum show reduced survival, indicated by lower values of log(survivors / non‑survivors).

• Such a fitness curve means that infants with very low or very high birth weights have a higher chance of death, whereas infants with moderate birth weights have the highest probability of surviving.

This pattern is the hallmark of stabilizing selection, where intermediate phenotypes are favored and extremes are selected against.

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Why the correct answer is stabilizing selection (Option D)

• Definition: Stabilizing selection favors individuals with intermediate trait values and removes individuals with extreme phenotypes from the population.

• Application to birth weight: In humans, very small babies are more likely to die due to underdevelopment, and very large babies increase complications during childbirth; hence, babies of moderate birth weight have the highest survival, producing a peak of fitness at the mean and lower fitness at both extremes.

• Match with graph: The inverted “U” curve—maximum survival at intermediate weight and lower survival at both low and high weights—fits exactly with stabilizing selection.

Therefore, the correct option is (D) stabilizing.

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Why the other options are incorrect

Option (A) directional selection

• Meaning: Directional selection favors one extreme of the trait distribution, shifting the population mean in one direction (either toward higher or lower values).

• Expected graph: Fitness would steadily increase or decrease with the trait; the curve would rise consistently toward one end of birth weight instead of peaking in the middle (for example, increasing survival with increasing weight only).

• Mismatch: In the question, survival is not highest at the smallest or largest birth weight; instead, it is highest in the middle and falls on both sides, so there is no one‑sided shift in mean birth weight.

Thus, directional selection does not describe the graph.

Option (B) disruptive selection

• Meaning: Disruptive selection favors individuals at both extremes of the trait distribution and selects against intermediates, often producing a bimodal distribution.

• Expected graph: Fitness would be low at intermediate birth weight and high at both very low and very high birth weights, creating a “U‑shaped” curve with two peaks.

• Mismatch: The graph shows the opposite pattern: survival is lowest at the extremes and highest at intermediate weights, forming an inverted “U”.

So, disruptive selection is clearly inconsistent with the given data.

Option (C) diversifying selection

• Meaning: Diversifying selection is usually treated as synonymous with disruptive selection: it increases variation by favoring two or more extreme phenotypes over intermediates.

• Expected graph: Like disruptive selection, fitness would peak at both low and high birth weights and dip in the middle.

• Mismatch: Again, the observed curve peaks at intermediate weight and declines toward both extremes, which is the exact opposite of a diversifying pattern.

Therefore, diversifying selection cannot explain the observed survival pattern.

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Summary for quick revision

• The graph of degree of survival vs birth weight is single-peaked with the highest survival at an intermediate weight and reduced survival at both extremes.

• This pattern is characteristic of stabilizing selection, making option (D) the correct answer for IIT JAM 2023 Biotechnology Question 29 on birth weight and survival.