31. Antibiotic resistance among bacteria represents
    (1) Balancing selection (2) Stabilizing selection
    (3) Directional selection (4) Disruptive selection

Introduction

Antibiotic resistance is one of the most pressing challenges in modern medicine. As bacteria evolve to withstand drugs designed to kill them, infections become harder to treat. But how does this resistance emerge and spread so rapidly? The answer lies in the evolutionary process known as directional selection.

What Is Directional Selection?

Directional selection is a type of natural selection where an environmental pressure consistently favors individuals with a specific trait, causing the frequency of that trait to increase in the population over time. In the context of bacteria and antibiotics, the “trait” is resistance to a particular drug.

How Antibiotic Resistance Demonstrates Directional Selection

When antibiotics are introduced into an environment—whether in hospitals, agriculture, or the community—they create a powerful selective pressure. Most bacteria are killed, but those with mutations or genes that confer resistance survive. These resistant bacteria then reproduce, passing their advantageous traits to the next generation. Over time, the population shifts toward increased resistance, a hallmark of directional selection.

Key Points

• Selective Pressure: Antibiotics kill susceptible bacteria, but resistant ones survive and multiply.

• Population Shift: The frequency of resistance genes rises, leading to a population dominated by resistant strains.

• Evolution in Action: This process is rapid and observable, especially in environments with heavy antibiotic use.

Real-World Impact

• Healthcare: Resistant bacteria like MRSA and multidrug-resistant E. coli are major threats in hospitals and clinics.

• Agriculture: Overuse of antibiotics in livestock accelerates the spread of resistance genes in the environment.

• Community Health: Resistant infections are harder and more expensive to treat, posing a global health risk.

Why Not Other Types of Selection?

• Balancing Selection: Maintains genetic diversity, but does not drive a consistent shift toward one trait.

• Stabilizing Selection: Favors average traits and reduces extremes, not applicable here.

• Disruptive Selection: Favors both extremes over intermediates, leading to two distinct forms, which is not the case with antibiotic resistance.

Conclusion

Antibiotic resistance in bacteria is a textbook example of directional selection, where environmental pressures favor individuals with a specific advantageous trait. Understanding this process is crucial for developing strategies to combat resistance and protect public health.

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Antibiotic resistance reminds us that evolution is not just a theory but a powerful force shaping life in real time—sometimes with serious consequences for human society.