Isolation | Zoology Optional Notes for UPSC PDF Download

Introduction

Isolation, in evolutionary terms, refers to the segregation of different populations into smaller units through specific mechanisms to prevent interbreeding among individuals.

Geographical Isolation

• Populations separated by geographical barriers such as rivers, seas, mountains, and deserts, or land and aquatic environments, are physically prevented from interbreeding.
• These populations, termed allopatric, evolve independently, accumulating genetic differences.
• The effectiveness of geographical isolation varies among species; for instance, small streams can act as barriers for land insects and mammals, while even mountains and oceans may not hinder birds from interbreeding.

Reproductive Isolation

• Prevents interbreeding in sympatric populations (living in the same area).

Premating Mechanisms

• Seasonal Isolation: Different breeding seasons prevent potential mates from coming into contact (e.g., plants with distinct flowering times).
• Habitat Isolation: Mates do not meet due to differences in habitats, food, space, or climate.
• Ethological Isolation: Behavioral differences, such as unique courtship displays, prevent mating.

Postmating Mechanisms

• Gamete Mortality: Mating occurs, but the egg is not fertilized.
• Zygote Mortality: Fertilized egg dies due to differences in chromosomes.
• Zygote Inviability: Hybrid develops but is weak and dies prematurely due to physiological disturbances.
• Hybrid Sterility: Hybrids are physically strong but sterile due to chromosome and gene arrangement differences.
• In rare cases, isolating mechanisms may break, leading to fertile hybrids, which can further crossbreed with parents, resulting in fertile offspring—a process known as introgression, leading to instant speciation.

Significance of Isolating Mechanisms

• Prevents wasteful courtship, ensuring only individuals of the same species engage in courtship.
• Protects the gene pool of a species and prevents hybridization.
• Avoids wastage of gametes and energy.
• Weak isolating mechanisms can lead to new species through hybridization.
• Absence of isolating mechanisms results in new species through instant speciation.
• Geographical isolation followed by reproductive isolation ultimately leads to the production of new species.
• Isolating mechanisms safeguard the identity of a species, a characteristic fiercely guarded by all species.