49. If natural selection favors an average phenotype by selecting against extreme variation, under such condition
    (1) Both alleles will coexist in population
    (2) Only dominant alleles will exist
    (3) Only recessive alleles will exist
    (4) Both the alleles will be lost

Correct answer:
(1) Both alleles will coexist in population

Introduction

Stabilizing selection is one of the most common forms of natural selection in nature. It acts to preserve the average phenotype and reduce extremes, but what does this mean for the underlying genetic diversity? Does stabilizing selection eliminate alleles, or can both remain in the population?

What Is Stabilizing Selection?

Stabilizing selection occurs when individuals with intermediate or average traits are more likely to survive and reproduce than those with extreme traits. This narrows the range of phenotypes and maintains the population mean over generations.

Genetic Consequences of Stabilizing Selection

• Reduction in Phenotypic Variation: The population becomes more uniform, with fewer individuals at the extremes.

• Allelic Coexistence: Both alleles can remain in the population, especially if the favored phenotype is produced by heterozygotes or if the trait is polygenic.

• Maintenance of Genetic Diversity: While stabilizing selection reduces phenotypic diversity, it does not necessarily drive alleles to extinction. Mutation, recombination, and balancing selection can help maintain allelic variation.

Real-World Example

In a population of mice, if stabilizing selection favors brown fur (the average), both the allele for lighter fur and the allele for darker fur can persist, provided that heterozygotes or combinations of alleles best match the favored phenotype.

Conclusion

When natural selection favors the average phenotype and selects against extremes, both alleles will typically coexist in the population. Stabilizing selection maintains the status quo and preserves genetic diversity, ensuring the population remains well-adapted to its environment.

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