40. Self-cross between Aa results in AA, Aa & aa F1 progenies, the force of evolution preserving either AA or aa is-
    (1) Stabilizing selection (2) Cyclic
    (3) Disruptive (4) Directional

Introduction

In evolutionary biology, natural selection can shape genetic variation in several ways. One powerful process is disruptive selection, which increases the frequency of extreme traits while reducing the presence of intermediates. This mechanism is key to understanding how populations diversify and potentially split into new species.

What Is Disruptive Selection?

Disruptive selection occurs when environmental or selective pressures favor individuals at both ends of the genetic or phenotypic spectrum, rather than those with intermediate traits. In the context of a self-cross between Aa individuals, disruptive selection would increase the prevalence of both AA and aa genotypes, while decreasing the frequency of the heterozygote Aa.

Key Features

• Favors Extremes: Both homozygous forms (AA and aa) are preserved.

• Reduces Intermediates: The heterozygote (Aa) becomes less common.

• Increases Genetic Diversity: Leads to a bimodal distribution in the population.

• Potential for Speciation: Over time, disruptive selection can lead to the formation of two distinct groups or species.

Classic Examples

• Darwin’s Finches: In environments where only large and small seeds are available, finches with either large or small beaks are favored, while those with intermediate beaks are at a disadvantage.

• Color Polymorphism in Moths: In regions with sharply contrasting backgrounds, both light and dark moths may be favored, while intermediate-colored moths are more visible to predators.

Disruptive Selection vs. Other Types

Evolutionary Impact

Disruptive selection is a driving force behind the emergence of diversity within populations. By favoring the extremes, it can create conditions ripe for speciation, especially if the two groups become reproductively isolated over time.

Conclusion

When evolutionary forces preserve either AA or aa genotypes after a self-cross, disruptive selection is at work. By favoring the extremes and reducing intermediates, disruptive selection plays a crucial role in increasing genetic diversity and driving evolutionary change.

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Disruptive selection is a key evolutionary process that shapes the genetic landscape of populations, often leading to greater diversity and the potential for new species to arise.