10. The relationship between birth weight and degree of survival in 6908 human births in an obstetric hospital is shown in the figure below. The mode of selection for birth weight is _____ .

(A) directional

(B) disruptive

(D) stabilizing

Mode of Natural Selection for Human Birth Weight and Survival

Understanding the Relationship Between Human Birth Weight and Survival

The relationship between human birth weight and survival is one of the most widely discussed examples of natural selection acting on a quantitative trait. Birth weight is a continuous phenotypic characteristic, meaning that individuals may show a wide range of values rather than falling into only a few distinct categories.

The figure described in the question is based on observations of 6908 human births in an obstetric hospital. Such data show that babies with an intermediate birth weight generally have the highest probability of survival, whereas babies at both extremes of the birth-weight distribution have a lower probability of survival.

Very low birth weight is associated with reduced survival because extremely small babies may have underdeveloped organs, reduced physiological reserves, difficulty maintaining body temperature, and other complications. At the opposite extreme, very high birth weight may also reduce survival because larger babies can face difficulties during childbirth and other complications.

Therefore, individuals with an intermediate birth weight have greater survival than individuals with either extremely low or extremely high birth weight. Natural selection consequently acts against both extreme phenotypes and favors the intermediate phenotype.

This pattern is known as stabilizing selection.

Why Is Stabilizing Selection the Correct Answer?

Intermediate Birth Weight Has the Highest Survival

Stabilizing selection occurs when individuals with intermediate phenotypes have higher fitness than individuals with extreme phenotypes. As a result, both extremes of a phenotypic distribution are selected against.

In the case of human birth weight, the relationship can be understood as:

Very low birth weight → Lower survival

Intermediate birth weight → Highest survival

Very high birth weight → Lower survival

The central or intermediate phenotype therefore has the greatest biological fitness. Since both extremes experience stronger negative selection, the population tends to remain concentrated around an optimum birth weight.

This is exactly the pattern expected under stabilizing selection.

The selection process can be summarized as:

Both extreme phenotypes selected against → Intermediate phenotype favored → Reduced phenotypic variation around the optimum

Therefore, Option (D) Stabilizing selection is the correct answer.

What Is Stabilizing Selection?

Stabilizing selection is a mode of natural selection in which intermediate phenotypes are favored over extreme phenotypes. It is especially common when a population is already well adapted to a relatively stable environment.

Unlike directional selection, stabilizing selection does not continuously move the population mean toward one extreme. Instead, it tends to maintain the existing average phenotype while removing individuals that deviate too far from the optimum.

In statistical terms, stabilizing selection generally reduces the variance of a phenotypic trait. The mean may remain approximately unchanged, but individuals become more concentrated around the optimal phenotype.

Human birth weight provides a classic example because neither the lowest nor the highest birth weights provide maximum survival. Instead, the highest survival occurs within an intermediate range.

How Does the Birth Weight Example Demonstrate Natural Selection?

Natural selection depends on differences in survival and reproductive success among individuals with different phenotypes. If a particular phenotype is associated with higher survival, individuals possessing that phenotype are more likely to survive and potentially contribute to future generations.

For human birth weight, survival is not equally distributed across the entire range of phenotypes. Babies at the extreme low end of the birth-weight distribution show reduced survival, while babies at the extreme high end may also experience reduced survival. Babies near the intermediate optimum show the greatest probability of survival.

The relationship between phenotype and fitness can therefore be represented as:

Extreme low phenotype → Low fitness

Intermediate phenotype → High fitness

Extreme high phenotype → Low fitness

This pattern produces a fitness curve with maximum fitness near the centre and lower fitness toward both extremes. Such a fitness pattern is the defining feature of stabilizing selection.

Detailed Explanation of Option (A): Directional Selection

Why Is Directional Selection Incorrect?

Directional selection occurs when one extreme phenotype has greater fitness than all other phenotypes. Under this mode of selection, the population mean gradually shifts toward the favored extreme.

For example, suppose individuals with increasingly larger body size consistently had higher survival than individuals with smaller or intermediate body size. In that situation, natural selection would favor the larger extreme, and the average body size of the population could increase over generations.

The general pattern of directional selection is:

One extreme phenotype favored → Population mean shifts toward that extreme

The human birth-weight data do not show this pattern. The largest babies do not consistently have the highest survival, and the smallest babies are not favored either. Instead, both extreme birth weights are associated with reduced survival.

Since neither extreme is continuously favored, Option (A) Directional selection is incorrect.

Detailed Explanation of Option (B): Disruptive Selection

Why Is Disruptive Selection Incorrect?

Disruptive selection occurs when both extreme phenotypes have higher fitness than the intermediate phenotype. In other words, individuals at both ends of a phenotypic distribution are favored, while intermediate individuals are selected against.

The general pattern is:

Extreme phenotype 1 → Favored

Intermediate phenotype → Selected against

Extreme phenotype 2 → Favored

This type of selection may increase phenotypic variation within a population and can produce two distinct peaks in the phenotypic distribution.

The birth-weight example shows exactly the opposite pattern. Extremely small and extremely large babies have lower survival, while babies with intermediate birth weights show the highest survival.

Therefore, the pattern is not disruptive selection, and Option (B) is incorrect.

Detailed Explanation of Option (C): Diversifying Selection

Why Is Diversifying Selection Incorrect?

Diversifying selection is commonly used as another term for disruptive selection. It refers to a pattern in which individuals with extreme phenotypes have greater fitness than individuals with intermediate phenotypes.

Under diversifying selection, different extreme phenotypes may be favored under different ecological conditions. This can increase variation within a population and may lead to the development of distinct phenotypic groups.

The general relationship is:

Both extremes favored → Intermediate phenotype selected against

In the human birth-weight example, both extremes are not favored. Instead, babies with intermediate birth weights have the highest survival.

Therefore, Option (C) Diversifying selection is incorrect.

Detailed Explanation of Option (D): Stabilizing Selection

Why Does Stabilizing Selection Best Explain Human Birth Weight?

Stabilizing selection favors an intermediate phenotype and eliminates or reduces the frequency of extreme phenotypes.

Human birth weight follows this pattern because babies of intermediate weight have the highest survival probability. Extremely low birth weight can be associated with physiological and developmental challenges, while extremely high birth weight can create complications that also reduce survival.

As a result, natural selection acts against both ends of the distribution.

The complete pattern is:

Low birth weight → Selected against

Intermediate birth weight → Favored

High birth weight → Selected against

This selective process maintains the population around an optimal birth-weight range and reduces the frequency of extreme phenotypes.

Therefore, Option (D) Stabilizing selection is correct.

Stabilizing Selection Versus Directional and Disruptive Selection

The three major modes of natural selection can be distinguished by identifying which phenotype has the highest fitness.

In directional selection, one extreme phenotype is favored, causing the population mean to shift toward that extreme. In disruptive or diversifying selection, both extreme phenotypes are favored and the intermediate phenotype is selected against. In stabilizing selection, the intermediate phenotype is favored and both extremes are selected against.

The human birth-weight example clearly follows the third pattern. Survival is highest at intermediate birth weights and lower at both extreme ends of the distribution.

Therefore, identifying the phenotype with maximum survival is the key to determining the correct mode of selection.

Effect of Stabilizing Selection on the Population

Stabilizing selection generally maintains an established optimal phenotype within a population. Since individuals near the average have greater fitness, extreme phenotypes contribute less to future generations.

Over time, this mode of selection tends to reduce phenotypic variation around the population mean. The mean value itself may remain relatively stable because selection is acting against deviations in both directions.

This is why stabilizing selection is often associated with traits that have an optimal intermediate value. Too little of the trait may reduce fitness, but too much may also be disadvantageous.

Human birth weight is an excellent example because maximum survival occurs within an intermediate range rather than at either extreme.

Final Answer

The relationship between birth weight and survival in humans represents stabilizing selection. Babies with intermediate birth weights have the highest degree of survival, whereas babies with extremely low or extremely high birth weights have lower survival. Thus, natural selection acts against both extreme phenotypes and favors the intermediate phenotype.

Therefore, the correct answer is (D) Stabilizing selection.