- The body weight of adult female of a strain ofDrosophila is 1.8±0.45 mg (mean ± standard deviation).In a laboratory experiment, each of the 3 groups (A, B, C) of this strain was subjected to a different type of selection pressure having influence on the female body weight. After many generations of experimental selection pressure, the body weight changed as follows:
Group A: Body weight distribution – Normal, 1.8 ± 0.08 mg
Group B: Body weight distribution – Bimodal at 1.4 and 2.2 mg
Group C: Body weight distribution – Normal, 2.2 ± 0.08 mg
Which of the following correctly gives the types of selection that have occurred in the three groups?
(1) Group A: Directional; Group B: Stabilizing; Group C: Disruptive
(2) Group A: Disruptive; Group B: Directional; Group C: Stabilizing
(3) Group A: Stabilizing; Group B: Disruptive; Group C: Directional
(4) Group A: Directional; Group B: Disruptive; Group C: Stabilizing
Introduction
Natural selection is the driving force behind evolution, shaping the traits of populations based on environmental pressures. In laboratory experiments, scientists can observe how different selection pressures influence traits like body weight in Drosophila (fruit flies). By analyzing the changes in mean and distribution of body weight, we can identify which type of natural selection has occurred: stabilizing, disruptive, or directional.
Let’s examine a classic experiment in which three groups of Drosophila were subjected to different selection pressures, resulting in distinct patterns of body weight distribution.
The Experimental Setup
Initial Population:
-
Mean body weight: 1.8 mg
-
Standard deviation: ±0.45 mg
After many generations:
-
Group A: Body weight distribution is normal, 1.8 ± 0.08 mg
-
Group B: Body weight distribution is bimodal at 1.4 mg and 2.2 mg
-
Group C: Body weight distribution is normal, 2.2 ± 0.08 mg
Analysis of Selection Types
Group A: Stabilizing Selection
-
Observation: The mean body weight remains at 1.8 mg, but the standard deviation drops from 0.45 mg to 0.08 mg.
-
Interpretation: The population’s body weight variation has dramatically decreased, with most individuals now close to the mean. This is a hallmark of stabilizing selection, which eliminates extremes and favors intermediate traits.
Group B: Disruptive Selection
-
Observation: The body weight distribution is now bimodal, with peaks at 1.4 mg and 2.2 mg.
-
Interpretation: The population has split into two groups at opposite extremes, with fewer individuals near the original mean. This is a classic result of disruptive selection, where both extremes are favored over the intermediate phenotype.
Group C: Directional Selection
-
Observation: The mean body weight has shifted from 1.8 mg to 2.2 mg, with a tight distribution (±0.08 mg).
-
Interpretation: The entire population has evolved toward a new, higher body weight. This is characteristic of directional selection, where one extreme phenotype is consistently favored, shifting the population mean in that direction.
Summary Table
| Group | Body Weight Distribution | Selection Type |
|---|---|---|
| A | Normal, 1.8 ± 0.08 mg | Stabilizing Selection |
| B | Bimodal, 1.4 & 2.2 mg | Disruptive Selection |
| C | Normal, 2.2 ± 0.08 mg | Directional Selection |
Correct Answer
(3) Group A: Stabilizing; Group B: Disruptive; Group C: Directional
Why Do These Patterns Matter?
Understanding these types of selection is crucial for predicting how populations respond to environmental changes:
-
Stabilizing selection maintains the status quo and reduces genetic diversity.
-
Disruptive selection increases diversity and can lead to speciation.
-
Directional selection drives evolutionary change toward a new optimum.
Conclusion
This Drosophila experiment beautifully demonstrates how different selection pressures lead to distinct evolutionary outcomes. By recognizing the patterns of stabilizing, disruptive, and directional selection, we gain deeper insight into the mechanisms that shape biodiversity and adaptation.
SEO-Friendly Slugs:
-
natural-selection-types-drosophila-body-weight-experiment
-
stabilizing-vs-disruptive-vs-directional-selection
-
drosophila-selection-experiment-explained
-
evolutionary-selection-patterns-in-lab
-
how-natural-selection-shapes-populations
Natural selection is not a one-size-fits-all process; it is dynamic, multifaceted, and central to the story of life’s diversity.
Correct answer:
(3) Group A: Stabilizing; Group B: Disruptive; Group C: Directional
