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UPSC MainsZOOLOGY-PAPER-II202510 Marks150 Words
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Q5.

Write on the following in about 150 words each: 1.(e) Sympatric and allopatric speciation

How to Approach

The question asks to write on sympatric and allopatric speciation, each within approximately 150 words. The approach will be to first define speciation briefly. Then, for each type (sympatric and allopatric), define it clearly, explain its primary mechanism, and provide a distinct example. A comparative table can be a good way to highlight the key differences efficiently within the word limit for competitive exams.

Model Answer

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Introduction

Speciation is a fundamental evolutionary process where one ancestral species diverges into two or more distinct species, unable to interbreed and produce fertile offspring. This divergence results from the accumulation of genetic differences, driven by various evolutionary mechanisms. The primary distinction among modes of speciation often lies in the geographical relationship between the diverging populations. Historically, Charles Darwin first described the role of natural selection in speciation in "On the Origin of Species" (1859). Two widely recognized modes are allopatric and sympatric speciation, differentiated by the presence or absence of a geographical barrier.

Allopatric Speciation

Allopatric speciation (from Greek "allos" meaning "other" and "patra" meaning "homeland") occurs when a population is divided into two or more geographically isolated subpopulations by a physical barrier. This barrier prevents gene flow between the groups, leading to reproductive isolation. Over time, each isolated population undergoes independent evolutionary changes due to different selective pressures, random mutations, and genetic drift. These genetic divergences eventually make them reproductively incompatible, even if the geographical barrier is later removed. It is considered the most common mode of speciation in sexually reproducing organisms.

Sympatric Speciation

Sympatric speciation (from Greek "sym" meaning "same" and "patra" meaning "homeland") involves the formation of new species from an ancestral species while they inhabit the same geographical area. Unlike allopatric speciation, there is no physical barrier to gene flow. Instead, reproductive isolation arises through other mechanisms within the shared habitat, such as ecological niche differentiation, behavioral isolation, temporal isolation (mating at different times), or genetic factors like polyploidy (common in plants). It is a more controversial but increasingly recognized mode of speciation.

Comparison: Allopatric vs. Sympatric Speciation

Feature Allopatric Speciation Sympatric Speciation
Geographic Isolation Present (physical barrier) Absent (occurs in the same area)
Primary Mechanism Geographic separation leading to genetic divergence Ecological differentiation, behavioral changes, polyploidy
Gene Flow Prevented by physical barrier Reduced or restricted within the same area
Examples Darwin's finches, Grand Canyon squirrels Cichlid fish in African lakes, Apple maggot flies

Conclusion

Both allopatric and sympatric speciation contribute significantly to the rich biodiversity observed today. While allopatric speciation, driven by geographic isolation, is widely accepted as a predominant mechanism, sympatric speciation, though initially debated, is now increasingly evidenced, especially with advancements in genetic and ecological studies. Understanding these distinct modes of speciation is crucial for comprehending the evolutionary history of life and predicting future patterns of species diversification in response to environmental changes.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Speciation
The evolutionary process by which populations evolve to become distinct species, reproductively isolated from their ancestral population.
Reproductive Isolation
The inability of a species to interbreed successfully with related species, due to various pre-zygotic (e.g., habitat, temporal, behavioral) or post-zygotic (e.g., hybrid inviability, sterility) barriers.

Key Statistics

Approximately 47% of all angiosperms (flowering plants) are polyploids, a common mechanism for rapid sympatric speciation in plants.

Source: IASZoology.com (citing White, 1978)

Examples

Darwin's Finches (Allopatric Speciation)

The Galapagos finches are a classic example. An ancestral finch population arrived on the islands, and as populations colonized different islands and adapted to their unique environments (e.g., different food sources), they evolved distinct beak shapes and became reproductively isolated species.

Cichlid Fish in African Rift Valley Lakes (Sympatric Speciation)

In lakes like Lake Victoria, hundreds of cichlid fish species have evolved. Within the same lake, different species exploit various ecological niches (e.g., different food sources or depths), leading to sexual selection and reproductive isolation based on mating preferences.

Frequently Asked Questions

▶Is sympatric speciation less common than allopatric speciation?

Traditionally, allopatric speciation has been considered the dominant mode, especially in animals. However, recent research and a better understanding of mechanisms like ecological differentiation and polyploidy suggest that sympatric speciation might be more common than previously thought, particularly in plants and certain animal groups like insects and fish.

Topics Covered

EvolutionSpeciationEvolutionary Mechanisms
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