Introduction

In the world of evolutionary biology, not all changes push species toward new extremes. Sometimes, the most successful strategy is to stay close to the average. This is the essence of stabilizing selection, a common and powerful form of natural selection that eliminates extreme phenotypes and maintains the status quo in populations.

What Is Stabilizing Selection?

Stabilizing selection is a type of natural selection that favors individuals with intermediate or average traits, while selecting against those with extreme characteristics. Over generations, this process reduces the prevalence of extreme phenotypes, resulting in a population where most individuals cluster around the mean.

Key Features

• Favors the Average: Individuals with moderate traits are more likely to survive and reproduce.

• Eliminates Extremes: Both ends of the trait spectrum—too high or too low—are selected against.

• Reduces Variation: Genetic diversity decreases as extreme traits become rare or disappear.

• Maintains Adaptation: Traits that are already well-adapted to the environment are preserved.

How Does Stabilizing Selection Work?

Imagine a population of birds where clutch size (number of eggs laid) is under stabilizing selection. Birds laying too few eggs may not pass on enough genes, while those laying too many may not be able to care for all their chicks. Over time, birds with an intermediate clutch size have the highest fitness, and the extremes are gradually eliminated.

Classic Examples

• Human Birth Weight: Babies with very low or very high birth weights have higher mortality rates, so average-sized babies are more likely to survive and reproduce.

• Bird Egg Clutch Size: Too few or too many eggs both reduce reproductive success, so the average clutch size is favored.

• Mouse Fur Color: In a stable environment, mice whose fur best matches the background are less likely to be eaten, while those with much lighter or darker fur are more visible to predators.

Stabilizing Selection vs. Other Types

Evolutionary Impact

Stabilizing selection is crucial for maintaining evolutionary stability in populations living in consistent environments. It ensures that well-adapted traits persist, while extremes that might reduce fitness are weeded out. Over long periods, this can lead to little change in a species’ appearance—explaining why some organisms, like horseshoe crabs and sharks, have remained relatively unchanged for millions of years.

Conclusion

Stabilizing selection is the evolutionary process that eliminates extremes and favors the average, maintaining the most adaptive traits within a population. This form of selection is essential for the long-term stability and survival of species in stable environments.

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Stabilizing selection shows that sometimes, in evolution, being average is the best way to thrive.