PRELIMS
Organic Evolution
Evolution is the gradual change from lower to higher categories of life forms over geological time.
Anatomical Evidence
Homologous Organs
- Similar structure/origin; different function
- Examples: Whale flipper, Bat wing, Horse forelimb, Human hand
Analogous Organs
- Different structure/origin; similar function
- Examples: Butterfly wing vs. Bird wing
Vestigial Organs
- Functionless in current form; functional in ancestors
- Examples: Vermiform Appendix, Nictitating membrane in humans
Embryological Evidence
Von Baer's Law / Embryological Recapitulation: Early embryos of different vertebrate classes (fish, reptile, bird, mammal) show striking similarities (e.g., gill slits, tail) that gradually disappear as development proceeds — suggesting a common ancestry. Popularized as the Biogenetic Law ("Ontogeny repeats Phylogeny") by Ernst Haeckel.
Origin of Life (Note)
- Oparin-Haldane Theory: Proposed that life originated from simple inorganic molecules in Earth's primitive reducing atmosphere (chemical evolution), given energy from lightning/UV radiation.
- Miller-Urey Experiment (1953): Simulated primitive Earth conditions (methane, ammonia, hydrogen, water vapour + electric spark) and synthesized simple amino acids, supporting the Oparin-Haldane hypothesis.
Fossil Evidence
- Fossils: Remains of ancient life (studied in Paleontology).
- Archaeopteryx: Connecting link between Reptiles and Birds (Aves). Has features of both.
Major Theories of Evolution
- Lamarckism (Inheritance of Acquired Characters):
- Proposed by Jean Baptist Lamarck (Philosophic Zoologique).
- Focus: "Use and Disuse" of organs. Acquired traits are inherited.
- Criticism: Strongly criticized by Weismann (Germplasm theory).
- Darwinism (Natural Selection):
- Proposed by Charles Darwin (The Origin of Species, 1859).
- Focus: Enormous fertility, struggle for existence, survival of the fittest, and natural selection.
- Modern Synthetic Theory:
- Current accepted theory. Combines Darwinism with genetics (Genetic variation + Natural selection).
UPSC Relevance
The "Who and How" of evolution is a frequent Prelims check.
- Morphological Differences: Differentiating between Homologous (common ancestor) and Analogous (convergent evolution) is a standard test point.
- Connecting Links: Knowing Archaeopteryx (Reptile/Bird) and the concept of vestigial organs like the appendix involves fundamental S&T basics.
- History of Science: Identifying the seminal works of Lamarck and Darwin link Biology to broader Scientific Revolution contexts.