50. Selection in which two extreme phenotypes leave more offspring than the intermediate phenotype is called
    (1) directional selection. (2) disruptive selection.
    (3) stabilizing selection. (4) neutral selection.

Correct answer:
(2) disruptive selection.

Introduction

Natural selection is a powerful evolutionary force that shapes the diversity of life. While most people are familiar with selection that favors the average or a single extreme, disruptive selection is a fascinating process that works in the opposite way: it promotes the success of individuals at both extremes of a trait, while selecting against those with intermediate characteristics.

What Is Disruptive Selection?

Disruptive selection (also called diversifying selection) is a form of natural selection where individuals with extreme values for a trait have higher fitness than those with intermediate values. Over time, this leads to an increase in the frequency of extreme phenotypes and a decrease in the number of intermediates.

Key Features

• Favors extremes: Both ends of the trait spectrum are selected for.

• Selects against intermediates: Individuals with average traits produce fewer offspring.

• Increases variation: The population becomes more genetically and phenotypically diverse.

• Can lead to speciation: With enough time and reproductive isolation, disruptive selection can split a population into two distinct species.

Classic Examples of Disruptive Selection

• Peppered moths: In polluted industrial areas, dark-colored moths survive better, while in rural areas, light-colored moths have the advantage. Intermediate-colored moths are less fit in both environments, resulting in a bimodal distribution.

• Darwin’s finches: On the Galápagos Islands, some finches have large beaks for cracking seeds, while others have small beaks for eating insects. Birds with intermediate beak sizes are less efficient at both tasks and are selected against.

• Mexican spadefoot toad tadpoles: Tadpoles with either omnivorous or carnivorous diets thrive, while those with intermediate diets face more competition and lower survival.

How Disruptive Selection Works

Disruptive selection often occurs when environmental conditions or resource availability create distinct niches that favor different adaptations. For example, if only very large and very small seeds are available, birds with corresponding beak sizes will thrive, while those with medium beaks will struggle to compete.

Graphical Representation

A typical disruptive selection graph shows two peaks at the extremes of the trait distribution, with a dip in the middle representing the reduced frequency of intermediate phenotypes.

Evolutionary Impact

• Genetic diversity: Disruptive selection increases the range of traits in a population, making it more adaptable to changing environments.

• Potential for speciation: If the two extremes become reproductively isolated, this process can lead to the formation of new species.

Conclusion

Disruptive selection is the evolutionary process that favors individuals at both extremes of a trait, increasing diversity and driving the potential for new species to arise. By selecting against intermediates, it shapes populations in unique and dynamic ways.

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Disruptive selection is a powerful force in evolution, reminding us that sometimes, being different is the key to survival and success.